It would be a major help to have the instructions available for review.
Here is a link to one such product: http://www.plumbingstore.com/clear-mirror-installation.html#shower
UL lising or not, I see this as a code-compliant installation only if the mirror is in a damp location (over the sink, not in the shower), and the mirror is removable.
Note that the instructions are silent as to reattaching the mirror to the wall. IMO, the common practice of attaching the mirror with caulk would not be allowed, as it would make the box inacessable.
The access issue remains the same whether the wire are low voltage or line voltage.
IMO, the mirror needs to be GFCI protected. It is common for bath lights to NOT be GFCI protected. This sets the stage for an unprotected mirror, as they want you to tie into the light feed.