Well, this guy at overclockers used a similar system to the P4 mounts on a Socket A for his HSFs, but improved upon it:
http://www.overclockers.com/tips562
But for waterblocks, what about puting the 4 bolts in so the head is on the back of the motherboard. Then put a nut on the top of the motherboard to hold the bolts in. A waterblock could then slide on and off without having to remove the bolts and motherboard. I suppose you could use a hold down plate with a nut on top, or maybe work a spring in there:
Code:
NUT NUT
/ /
\ \ <- SPRINGS
/ /
___________________________ <- HOLD DOWN
-------------------------
| WATERBLOCK | <- BOLT THRU WATERBLOCK
-------------------------
NUT [=====CPU=======] NUT
=========================== <- MOTHERBOARD
[_] [_] <- BOLT HEADS
I hope that's not a confusing diagram above