My only concern would be the UV light possibly reflecting off the mirror and curing ink on the bottom of the carriage/heads. Whether that would actually happen or not, I'm not sure. I wasn't brave enough to try when we had a customer request a direct print to mirror. Instead, we ended up printing second surface to a piece of glass, then placing that over a mirror to give the illusion that the print was actually on the mirror surface. It turned out really well.