How thick is the poured concrete floor that is used for radiant floor heat? I can't imagine it would be much thicker than 2". How are those floors prepared for a concrete slab? Surely there is a way to preserve the solidity of such a small (shower dimensions?) area.
Wouldn't the creasemarks of the stamping application help avoid cracks?
"Contractors call these control joints. To the average homeowner these joints look like decorative lines. The principal is to make the slab thinner by cutting a line in the slab or by removing actual material so that the concrete doesn't crack randomly." --http://www.askthebuilder.com/364_Minimize_Cracks_while_Pouring_Concrete.shtml
Wouldn't suspending steel rebar in the slab reinforce it significantly?
Considering that the supporting floor members in question have already been reinforced, can they also be stabilized to minimize movements that might encourage cracking?
How much deflection are we talking about, on such a small area, indoors (not subjected to outdoor freezing).
On a negative note, sealing water out would be a maintenance issue, but certainly possible. This quote is worth noting, "The force of water cascading from the shower-head will quickly erode a finish layer of cement paste on the floor and reveal the rough texture of the fines (sand). Over time, the exposed layer of cement will trap dirt—whereas, the aforementioned finish materials (stone, tile, mosaic) will be able to withstand the relentless water erosion."
I love your idea JDenise, and I really want you to find a way to make it work. Here are some concrete sites I have come across in my researching:
From what I've read, it's important that you have someone who has been trained specifically in stamping concrete. That may be why your avg concrete guy would scratch his head, or even scoff at your suggestion. You should consider taking a class yourself, or at the very least investing in some books on the topic.
I wish you the best of luck, and please keep us updated!