As you probably already know, tiling over plywood is a bad idea. Self leveling compound is more of an underlayment and over plywood the only thing I would install is carpet.

I would install 1/4" hardie board. Set the mats and apply a layer of thinset. Let dry then more thinset and tile.

I have heard conflicting opinions about tiling over plywood but those that say it can work fine also say you need to follow specific guidelines such as using thinset with lots of latex. Since I wrote the post above I am leaning toward using the self leveling compound. My thinking was that the self-leveling compound would serve essentially the same purpose as hardie board and have the advantage that the heating mat would be embedded in the cured SLC leaving a smooth and level surface upon which to spread thinset and lay tile. If my logic isn't sound please let me know as I plan to do this saturday!

I would not set tile over plywood. I have been a remodeler for 35 years and I can truthfully say that my dream demo jobs are when I am removing tile floor from a plywood base. A couple good wacks with a hammer and everything pops up. Saves me a lot of labor and money. But if you insist, You would be wise to prime the plywood and forget about the slc. Embed the mat in thinset, let dry and then set tile with more thinset.

Don, so if I understand what you are saying - even if I use the SLC it wouldn't work as well as the hardie board. And the reason is that because it doesn't bind to the plywood as well? Even with hardie board you are putting thinset directly on plywood. The only difference would be the screws that hold the hardie board on. Or is there another reason why the hardie board approach is better? I will take your advice if I understand the basis for doing it your way - thanks.

What I see after 35 years of interior remodeling is what works and what doesn't. Often the product was fine but the installation was not and conversely the product was bad and install good. Usually its a combination of the two.
There are slcs' that may have the compressive strength and polymers to work in your situation and they have very specific instuctions as to priming and mesh. You will not find them at HD or Lowes. Those are generally for over concrete or as an underlayment for carpet.
My objective is to give you my opinion on what works, is accessible and diy. Others may disagree with this technique but I have never had a disatified client and my business is 100% repeat and referred customers.
Use a quality subfloor adhesive and 100% glue down of 1/4" hardi board. Screws every 4". Lay the mat as instructed and pour thin layer of thinset over entire floor making sure it stays flat, which will require some defferent size screed boards.Another way to keep it level and smooth would be to apply with a 1/4" notched trowel to maintain height, let dry then fill in the grooves. Let it dry overnight and then set your tile.
Don't mean to sound obtuse here but if you feel totally qualified in using the proper slc and all it entails you will probably be fine.

Well, I took your advice and installed the 1/4 inch backerboard and plan to embed the heated floor mat in thinset tomorrow. Thanks for describing the procedure step by step. The one part where you lost me is when you said "pour a thin layer of thinset". I assume by thin you mean thin mixed with more liquid than usual. I'm using thinset that is mixed with a latex liquid. I'm a little worried about deviating very much from the mixing directions. Would that affect the strength of the thinset? If I go with the 2nd approach what would be the reason for filling in the grooves? Is this step added to bring the floor closer to being consistently level i.e. by filling in the low areas?

Mix thinset as directed or you will absolutely have weak thinset.

By thin I meant thickness or about 1/4", just enough to cover the mat. If you cant eye ball it with a trowel I would suggest tacking 1/4" plywood strips on the floor spaced parallel to each other and some distance apart. Strike off the thinset between the strips. When it sets up some remove the strips and fill the voids in. You might want to babysit the thinset and once you can walk on it it should still be soft enough to scrape off bumps and high spots. I am better at doing than saying. I have great respect for teachers.

I wish I'd seen this sooner.

I'd have recommended installing your heating mat over the plywood. Then block off areas that need to be blocked off to contain your SLC. Then use a garden sprayer to spray SLC primer on the mat and subfloor, using a roller to get it all over. Then pour your SLC. Then tile over the SLC.

But no big deal.

The 1/4" cement board is fine too. If you have already installed the 1/4" cement board, hopefully it was bedded in thinset and fastened down. That's the only way it should ever go down, over a bed of thisnet. Then I'd install your heating mat, and because this is a pennyround installation I'd then use SLC primer, then SLC, then tile.

I'd never recommend to try to float a floor level and in plane for a mosaic tile installation with thinset. Thinset is a "thin setting" material. Put it on thick and it can shrink and crack. If you want a thicker bed, then use a medium bedding material instead of thinset.

A multitude of sins can be hid with larger tiles. But for small mosaic pennyrounds that will follow every contour of the setting bed, I highly recommend using SLC instead of trying to float the a floor flat with thinset and a trowel.

My opinion.

There are only a few slcs' that would work on wood or Hardi and tile and you may not find them at the big box stores. And God forbid you ask for help there. I have torn out more than my share of slc' over plywood I believe slc have been used in too many applications as a be all end all.
Applying a thin layer of thinset over dampened Hardi to cover the mat is fine. A bit more hands on technique involved though.
I have glued down Hardi or wonderboard for as long as they have been on the market and I will match my satisfied clients with anyone else.

Thanks guys for the interesting discussion. It looks like there are several paths to a successful installation but I think I will go with the SLC approach to embed the heat mat on top of the hardieboard (with primer appled first).

However, I have another bathroom with the identical situation (currently two layers of plywood and plan to install a heat mat under tile) that I will start on after this one is finished. The only difference is that I will be using 12x12 floor tiles rather than penny rounds. Can anyone suggest a type of SLC/primer combination that I could install directly on the plywood to embed the mat and lay tile on top of? The local Tile Shop sells a product called Pro-level that comes with its own primer. If anyone has experience with this or could suggest another high quality SLC please let me know.