If you ride the WHW across Rannoch Moor, at Victoria Bridge you can follow the sign to the right marked Loch Etive – excellent track that drops away from the summit in a really long (and occasionally technical) descent. Follows Glen Kinglass to Loch Etive, but the track south is a landrover track that climbs/falls significantly and really take sit out of you. Comes out at Taynuilt (from where you can road ride back up to Glen Orchy).
The other side of the loch is easier (in the southern 2/3rds anyway), you could get a boat across from where Kinglass comes out. This chap: – Donald Kennedy, Loch Etive Cruises 01866 822430 – is based at Bonawe and is a helpful chap. You may be able to persuade him to take you across or down the loch. The western side though brings you out at Bonawe quarry, you'd then need to riode the little road round to Connell Bridge to cross.
Not sure that really helps?