I attach the pulley to the side near the gear in such a way that there isn't enough room for the bushing to come off the fork, then I just hold the other side in place with a finger while truing the D124 tires. It takes a long time (up to 20 minutes[by contrast, a 1/32 car trues in about two minutes]) and is a bit of a pain if the bushing gets out of whack, but the results are fantastic.
I remember those photos of Harry's Razor and thought that was a great idea, but never equated it with being the fix to my D124 dilema. I'll have to give this a shot. Thanks for posting this, Jon!