Yup, that's on the white-board for a future release.
We realized shortly before release that it would have been a good feature to add. Unfortunately we couldn't be sure adding it wouldn't break something else (there is a lot of complexity with the way tracing papers are composited into the graphics pipeline, and the way paper textures are calculated).
So rather than putting in a broken implementation, we held off until we are sure we can do it without screwing something else up.
In the meantime, there is a quite 'rotate 180 degrees' on the '/' shortcut key. It's not as good as a mirror, but it at least breaks your visual process on the image enough to get an idea of problems with proportion.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"