Hi there , firstly it can't go onto a aluminium foiled backed mirror because it will conduct the heat and crack the mirror. Secondly if its just plywood and no other metal to conduct the heat then you could push it up against the plywood.

Also, why can't the mirror have aluminium foil backing when this is designed to protect mirrors in humid environments like a bathroom?

so if we use backer board (instead of ply) like Hardie Backer does the pad still need a tiny gap? I suppose what I am asking is does the pad need a tiny gap from any surface the mirror mounts on to?

Hi there , if the mirror has no aluminium foiled backing and is stuck onto to the back of the mirror and if you leave tiny gap just so the plywood is not touching the demister pad then it should be fine