So if you want to climb up an old, 'handholdy' church: When you walk up to the base of the church (presuming that your climbing skill is very high) your will get the option to switch into climbing mode. In climbing mode players will not be able to use weapons, potions, spells, etc. Instead the player will use the right and left mouse button for each of the MC's hands in game. The player will aim at where they see a handhold and use the left and right mouse buttons to determine which hand to use in order to scale the building. For example, if you are mid-climb and see a series of handholds to your left, but none to your right, you obviously want to go to your left, right? XD So you grab the nearest handhold(which is to your left) with your RIGHT hand, to give you momentum to reach the other handholds. You follow?
That makes sense, but whether it should be done depends on how much people want to make a minigame out of it. It could be a simple as go to wall, climb the wall. I happen to like your idea, though I'd want multiple paths up.