I expect that preventing a 51% style attack on any alt coin used will be necessary?
So that would mean that any coin used in the sidechain would have to be a coin that is potential competition for BTC too?
51% attacks only affect blockchains, not assets/"coins".
Ok so lest say "donkeycoin's" blockchain was being used, and there was a 51% attack on "donkeycoin". What are the possible implications?