A buddy has the 90BT and yes, it will play DVD's with the bypass while driving. But after I showed him what I do with my 700, he doesn't play DVD's anymore. I bought a 16GB SD and filled it with divx copies of movies set to 640 format for full screen, and I have 20 movies in my car all the time, no dics to store, and no load up time. Just thought I would pass that on in case you found it useful.
Can't answer your WRX question, however, I can't imagine the vehicle has anything to do with a bypass solution. Works the same on a bench as it does in a car/boat/motorcycle/.....