Nice seven-month bump there, I assume you read the already suggested? :tongue.gif:
Anyway, your thinking of this exactly how I was. With all the rope suggestions, this would merge quite nicely, being it simply a re-skinned, unmoving water block that doesn't drown/put out fire/turn into stone with lava. Although I don't believe you should be able to rappell, since that would be a bit strange looking and to implement. If ever implemented it should only be able to be shot upwards, IMO, with a max range of 15-20 blocks.
Although I can't say I support it, just giving out another opinion.
I would imagine the grappling hook would simply be a player tool and not actually spawn any in-world geometry. It could be hook-shot simple, simply point at a hookable block within range and it zips you there. Maybe once you're anchored, you can raise/lower before releasing.