I have not yet, but here's my thoughts off the dome:
Rolls are prone to failure, and it's best not to make very many of them- especially if you've got no choice beyond succeed or die. So, no to rolling MOVE every round. Perhaps only if they try the Move action twice.
1-handed weapons only. The other hand is gripping the walls/rope. (perhaps allow attacks with 2-handed weapons, but require a MOVE roll, and both the attack and MOVE are at -1 each).
I guess if they tied a harness out of a rope (SURVIVAL roll), they could use both hands, but movement would be severely restricted.
If they get hurt, or dodge, require a MOVE roll to hang on.
If a MOVE roll fails, the PC slips. Require another MOVE roll to catch hold of something. If that roll fails, I'd go with one of 2 options:
1) Be a hardass GM: PC falls all the way to the bottom.
2) Be a softy GM: PC falls, I dunno, 3D6 meters? 2D6? before hitting an outcropping, takes falling damage, and if not broken, give them another chance to roll MOVE to catch the ledge.
Thanks! This is close to what I have had in mind. I was thinking of splitting up the climb in stages, with eventual random encounters on each stage. Each stage would count as a quarter hour and require a MOVE roll. I agree that a failed MOVE roll should not result in the adventurer falling, but I have thought that maybe a pushed roll should. I am quite hardass and wouldn't have any trouble with letting an adventurer fall. This would lead to a direct modified roll for critical injuries, depending on how far down the adventurer has climbed (and how long the fall is).
Example: An adventurer tries to climb down a sinkhole. She fails the MOVE roll and cannot find a way down, unless she make a quite dangerous jump. The adventurer can take her time to find another way (try again the next quarter hour) or push the roll to go for the jump. If the pushed roll should fail, the adventurer falls to the bottom of the sinkhole and have to roll for a critical injury (blunt) with d6+60, as the fall is from the top of the sinkhole.