In reply to OwenF:
Damn! This is hard! I can imagine what you're looking for but I can't think of it! I've never looked at the place with a view to rigging an abseil. Looking at 1:25000 mapping you'll see the west coast has loads of cliffs - getting to them is the problem.
Heading west from Port charlotte you come to Kilchiaran and to the north there is a hill with radio masts. There are cliffs there overlooking the Atlantic but may be too big/inaccessible.
http://binged.it/12J0ka5
Following the track/path that runs round that hill to the north there is some bouldering to be had at grid 205 622. Or drive to Kilchoman (car park at 208 634) and walk south.
I'd be interested to know what you end up doing. Hope you have a good time anyway!
I'll post again if I have any brainwaves!