I don't think there is a code issue, but I think it would look weird. If you've done a good job on re-plumbing, how often are you going to be crawling around down there?

IMO, having access to plumbing is a good idea. Traps can clog, DWV lines can foul.
The floor covering would determine the ease of making an "invisible" hatch. If the flooring pattern contains contrasting color bands, easier yet. A suitable handle will be the hardest part of the design.
Can't comment on code issues, though I'm certain there are people who will tell you "can't be done".

Do you have a closet in the bathroom or one nearby where you could move your hatch?

Unfortunately I donÂt have a closet nearby to hide the hatch.

Handle:
For lifting the hatch, I thought I could leave a very small hole in the hatch (perhaps cover it with a plastic plug (like screws are hidden in furniture) and then use a screw whenever I want to pull the hatch out.

Making it "invisible":
The floor will be most likely tile. If the hatch borders coincide with the tile edges, I thought I could simply silicone caulk (rather than grout) the perimeter of the hatch. Having to simply recaulk every time I need the access does not seem too bad.
I would also like to have this hatch screwed to the floor with at least 4 screws, so making the screws "invisible" may be another challenge (perhaps again use screw plastic plugs like in furniture).

For the occasional inspection I do not really need this access; I can crawl (60Â) from the main hatch. But if I need to do any work at all, a closer hatch seems indispensable. I do not want to be in the dilemma of reopening the floor. Crawling in my house is complicated by the fact that the foundation uses end to end intermediate foundation walls (rather than piers).

Tile needs a very solid non-flexible surface to which to bond. Your hatch needs to remain lightweight and easily moved. The two needs counter each other out. If you build a hatch sturdy enough to not pop the tile off of it when it's walked on, then you'll need a crane to lift it. If the hatch isn't light weight and easily moved, the tile will crack and pop off.

I really can't think of a worse place for an access panel than the middle of a bathroom floor. Even the adjacent hallway would be better, as you could cover it with a rug and hide it.

Lots of houses in japan have storage hatches in the floor which are invisible when closed. How about making the sames a decorate border of contrasting tile? Here is a link to some ideas, but google "floor storage" for more-- only, of course, you'd be using yours for a hatch -- but the "door" would be the same.

Here is a link that might be useful: floor hatch

Thanks for the good suggestions.

live_wire_oak, Thanks for suggesting the adjacent hallway - good idea. It will be in the same proximity. Indeed thereÂs no need to put the hatch in the middle of the bathroom. Besides hallway may stay with existing carpeting which will completely hide the hatch. Just need to pull back carpeting to access. Not trivial but not difficult I imagine?