PA-22,
Good call on the panel wiring legality. I spoke with my AP yesterday about this, and he indicated that it was OK to hard wire the power cable direct to your aircraft wiring, as long as it is current protected (fuse or circuit brkr). You are correct that the unit is not permanently installed if you have a removable plug. I originally installed my gps this way, and it sure does make for a much neater install with no excess wire hanging. It also frees up the 12v outlet for other accessories.
Warrior Greg