I'm pleased to say the Demo went very well, thanks to an all night hacking session on Friday. I was able to address the GPS data lag and fuse GPS and gyro with a Kalman Filter.
Data Bus made 5 or 6 runs on Saturday, all successful. Position along the course was within about 2 meters.
The solution works well enough that even after crashing into one of the camera people, the robot still finished the course fairly close to the final waypoint.
Onto obstacle detection and avoidance!