I just got in the Smooth-on resin and supplies. I tried a few tests to see what kind of issues I'd run into using a resin like this in a water-based gel. I injected mixed resin into a shallow container of gel, mostly to make sure that the combination didn't cause major issues in curing, etc. The results were promising, if a little primitive. Here are some examples of what came out from simple injections:

The resin cured fine in a water-based gel, considering all the issues I've heard you can have with curing urethane resins in high moisture environments. So that's one issue I don't need to worry about.

I also found that, for the most part, injecting resin into a gel will allow some extrusion of shapes. That being said, I encountered two big issues I wasn't too thrilled about:

1. Resin Leaks occurred when I first tried to inject into the gel - the resin would travel up through the hole made by the needle and create a small pool at the surface of the gel.
2. The Resin Hardened Too Quickly as its pot time is only a few minutes. Those minutes go by fast. I think for future tests I'm going to need a resin with a longer pot time, and for the final proof of concept, I'll need to build something that mixes the resin just as it's going into the gel.

The resin leak issue was bugging me. The major issue was that the resin was too fluid, and the gel was too viscous. The gel held its shape too well, keeping holes open when it should have filled them in. The resin wanted to flow everywhere.

So I thought I'd try to test some different materials that I could tweak the viscosity of, to see if there was a balance of material to gel that worked well. The solution was to mix flour and water to act as the material, and whisk the gel with water to make it less viscous. And it worked great.

Now I could easily inject bubbles, lines, etc. into the gel. It was so much fun to extrude into the gel that I made some GIFs :)

Next steps are to get some more gel, some materials for thickening the resin, and some plaster. Wish me luck!