Well McLeod came through and shipped me a "slip on" throwout bearing. They even shipped it with Saturday delivery like I asked so I could get everything back together this weekend!
To say this bearing is higher quality than the Keisler one would be an understatement:
It's full of nice design features, like lines that swivel so you can get them into the bellhousing (as opposed to the Keisler setup where I had to use a crowsfoot and monkey around trying to attach the line through the clutch fork hole), is totally rebuildable, and even came with spare o-rings in case something goes wrong.
Of course, it also cost $350
.
At any rate I pulled the transmission out and everything put back together with the new bearing, and then took the Keisler bearing apart to see if I could figure out what happened to it. As far as I can tell, the piston cocked over and seized in the bore, causing the seal to blow out almost completely. I can't really tell because the piston was so seized that it was impossible to totally disassemble the thing.
So I've added another company to my "never again" list, but hopefully I'll be able to get the car running this week. Just need to pick up a 22" -4AN line to connect the McLeod bearing to the master cylinder.