The fact that you still had some oil in the tranny suggests that the drain plug didn't drop out till late in the ride, so little or no damage may have been done. That being said, it was professionally rebuilt and installed. Doesn't that mean that the professional you paid to do the work left the plug loose and is therefore responsible? If it was me, I would tell the shop that did the work about what happened, refill it and see if any problems arise. Drain it after the rally and see what is on the magnet. If it is clean, don't worry about it, but check it again after a few thousand miles. If you get a lot of metal on the magnet, they should rebuild it or replace it on their dime.