And I imagine on any phone with a face-facing camera (would that be "forward facing", or "rear facing"?) doing that with another application and a portable makeup mirror should be trivial.

All you need is an application that takes a picture without altering what is on the screen when you press both volume buttons, or something. Maybe even just takes a picture once a second for the next 30 seconds?

You could make an application explicitly for this purpose, and even build in the mirrored image logic. Only one person needs to do it, then every non-technical user can use it.