You could use SVG. Flipping text in Inkscape should be pretty straightforward... Object - Flip Horizontal (H). The text will even show up in searches, not just highlights. But some people frown upon using SVG in ePub/Mobi, probably because some e-readers (older Kindles, Sonys maybe?), run on dated software which simply can't handle vector graphics well. Works beautifully for PDF, tho.
Alternatively you could flip the actual glyphs within the font using FontStudio, TypeTool 3 (my favorite) or even FontForge, which is open-source. Running FontForge under Windows would probably be cleaner with VirtualBox and a Ubuntu LiveCD instead of messing with Cygwin and some 40.000 files... It may look fugly (
link), but it gets the job done, and it gets it done well. The down side is that you'll have to type them sdrawkcab (backwards) and that the glyphs won't be flipped anymore if the person reading it decides to use a different font.
Both methods allow you to use whatever font you want.

Perhaps even the ones you use throughout the book (for consistency).