I like the magnet idea. If you built brick-built horses with a fake barding that dragged the ground, you could hide the magnet under there. You could even hide minifig car wheels under there to help with performance.
On the underside of the raised playing field, you could have two rail road tracks, each with one train motor with a boom and a magnet attached. With the booms, you could suspend the magnets closer to center than you could if they were simply lifted above the two motors (since the LEGO train track would prevent it).
It'd be just like the train racers that were at Brickfest '03, except instead of head to head racing, it'd be head-on racing. You could design it so that the booms lead the horses to pass mere centimeters from each other.
Then you'd place your knight on the horse and a third person would count down to when you turned the train regulator switch.
To make the knight's fall off better, you could brick build their legs into the horse (or no legs at all) and attach the knight's torso by means of a jumper plate. The one stud holding the torso on would be a weak connection, encouraging a knock off blow (and only half a knight lieing on the ground would make for a satisfyingly gory outcome).
I'd also like to say that going to Brickfest this year is looking to be a greater and greater possibility for me. On top of that, if I play my cards right, I may be even able to build and bring the giant castle
I've planned. I think I could build it in three months time...
Of course with a 144+ x 144+ stud footprint, Magnus might tell me and my castle to get lost and find another room
And if I drive, then that would mean I could bring the red dragon and the temple and the griffon and the treant and the...