I will attempt to describe this round table top I did but it might not make sense. Sorry I don't have pictures. The table top the client wanted was too large to be cut from a single sheet of hardibacker. What they did was cut two half-rounds and lay them together with the seam going north-south and cut two more half-rounds and lay them together with the seam going east-west, and laid one set on top of the other set and glued and bolted them together into one solid unit. Then I created a mosaic using thinset as an adhesive and I finished the edges with thinset. The same concept would work with a square or rectangle table - just offset the seams. Also they used large washers with the bolts. Hope this helps.

Thank-you for your answer silvamae. I'm not sure that would work because there is a frame around the edge but nothing under where the glass would go. I should have mentioned it's a rectangular set and not a pedestal set!
Here's a link to a similar set, as you can see, there just isn't any support. It's a full size dining table. My friend left it to me when she passed away, and she wanted it for my garden. Unfortunately, the tempered glass has broken and she had it replaced with thick sheet glass which is not strong.

Here is a link that might be useful: wrought iron 50's table set

Here's another thought -- may not work but I think it will. Make your table top larger than the top frame of the table; have it extend past the frame several inches all the way around. Forget about trying to place it inside the framework. The weight of the mosaic and hardiback will hold the tabletop in place.

Been a while. Many things going on, but do check in. May I please offer a suggestion. Get 4 or 5 strips of sturdy metal and attach to the underside. The strips shouldn't raise your hardi too much, and it will keep the top inplace as well as giving you the support u need. Good luck. Send pix when done.

Actually, your combined ideas are fairly good. However the hardyboard does not come in large enough sheets, and I would not like a table top in two pieces.
My mosaic work is no where near as lovely as you all's is. Mainly I just use glass 1/2 rounds and nothing else. I tried using broken tiles etc. but I'm a spaz, and after cutting open hands AND FEET (the tops of feet, don't even ask!) my DH has forbade any more broken pottery of any kind. He doesn't say much, but he said it would surely help him with his sanity.