What I would do (and did kind of) is create pockets in the background that fits a small amount of substrate, and put the plant in there. Like, in the styrofoam, make a hole/dent in it the size of your thumb, and then smother it in concrete/drylock, and you have a plant cubby.
Yes, I imagine plant roots could go through the concrete and root in the styrofoam given enough time... but that doesn't really cause any issues.
The Styrofoam will not deteriorate if it gets wet, and its not toxic and won't hurt your tank if its exposed. The concrete/drylock covering on the foam is mostly cosmetic.
I made a small plant shelf in my tank, which is working well. I don't really have a good picture of it to show you, but its basically the same concept as what I described above but mine is about 2 inches X 1 inch deep X 8 inches wide. So far it looks like the plant roots aren't penetrating the concrete, they're just kinda branching out on top of the substrate.
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