You wouldn't have to cut any background away since the GS and silicone are waterproof. In one of my dart tanks I actually pulled back the hydroballs and substrate, laid a piece of screen material in and used large stones to create a water feature which is pretty much only the collected water from the misting system. This made an easy place for me to remove the collected water.

To do so with tree frogs, just use large enough stones so the frogs can't swallow them.
The frogs you are planning on are not the most aggressive eaters so large aquarium stones are not something they are going to swallow. Just don't use pea gravel or small aquarium stones.

To change the water, just simply use a siphon hose and replace the water with fresh every few days. Only takes minutes.