MH152: I think you're being rather pessimistic rejecting anything less than 700m. But don't take my word for it - take a look at your perf charts and you'll see 500m is possible in many conditions in a full 172.
Shortest crossing from Skye to Benbecula is only 13nm so if it's a nice day there's plenty scope for just going high enough to glide to the other side in the event of donkeys taking the huff. But having got down to Barra you will probably want to go direct to Tiree so I'd echo crashondeck and say take survival suits. Only place I know that rents them is Andrew Bruce at Far North Aviation in Wick which is probably a bit out of your way. I also have some which I'd be prepared to rent to you if you'd like to pm me.
NS