i love the idea because its very secure just in case u have visitors or nosy people that like to poke around in the bathroom.
it will always be there. , and hopefully will work when needed...maybe stash two guns( per mirror ) in there if possible...or a revolver or something.
cons..well if shes hiding from the BG, and she she breaks the mirror, that'll give away where shes at..and if she waits for the last mins to break the glass, then she might not get to it in time, so maybe a reinforced bathroom door might help.
second con, is the glass might put her in danger, if it ends up on the floor, she might step on it. ( well i dont wear shoes in the house. so this might be diff for you)
and as someone mentioned, she might cut her hand...maybe a hidden hammer under the sink or something might work better =]