From an artist point of view, I would try this...
On the lower half of the mirror I would caulk flat slate, or stones with flat sides. This should reduce the amount of time your frogs spend looking in the mirror. (Try it on your bathroom mirror first... if your wife is out of there within an hours time, then it worked!) On the upper half of the mirror, from a frogs perspective, they will only see more plants, and the reflection of the inside/top of your tank. While your house guests see what appears to be a really deep tank, (Horizontally deep, not vertically deep.).
Alternately you could make a bamboo or bark wall to hide the lower half of the mirror, topped off with a thin, full width mini planter for a creeping plant. Like a PVC pipe that is cut in half and coated with aquarium caulk inside and out, also fastening sand or small stones to the outside of the PVC to mask the caulking...
(Do not use paints unless they are made for aquarium/terrariums and don't use PVC without covering every square inch with caulking. PVC releases highly toxic chemicals as it degrades.)
You may also think about affixing "U" shaped glass or plastic planters on the upper half of the mirror for an invisible hanging planter effect. The half planter will look like a full planter that is just floating. Caulk it right to the mirror, or get one with suctioncups... Personally, I would caulk it to the side and possibly drill a small hole to allow excess water to drain, preventing root-rot, or leave the bottommost portion uncaulked, with a paper thin gap where the glass touches the mirror.
Also, use caution when purchasing mirrors, cheap mirrors are made with surface plate materials that develop black spots, and web-lines over time in any environment. If it is an option, make sure that you get a moderatly priced BATHROOM mirror which should have a rather thick and rough grey coating on the back of the mirror which is moisture resistant. Be sure to use a non-hardening glue to affix the mirror to your back-board medium, Liquid-Nails makes a mirror glue, or semihard glue that will flex when needed, remember, wood swells and shrinks with humidity... Translation, full length mirror cracks.