Re: Grey Mantala for false Bottom
I put a layer of Glass Box Tropicals' "substrate barrier" (looks like landscape fabric to me) down on top of the Matala. I cut the barrier material about 2" larger than needed, and then folded the extra up against the sides of the viv, then backfilled with ABG to press the barrier up against the glass and seal around the edges of the Matala.
Your thought about siliconing and cutting holes for drainage suggests that you aren't imagining Matala correctly. It is a very open weave (think Brillo pad), so drainage through the material is like a coarse sieve (hence the need for barrier fabric).
On the water area, the substrate barrier simply follows the contour of the cut edge (I cut at an angle as best I could) down to the bottom glass, the the aquarium gravel holds the barrier down. The rocks cover the substrate barrier (for aesthetics) and serve as a bit of a wall to prevent ABG from washing into the pond (I don't think this would happen much -- ABG stays in place better than soil or the like).
You are correct to not allow frogs to come in contact with the Matala -- they'd likely get tangled in it.
I think I could turn and live with animals, they are so placid and self-contain'd.