Any chance the BTC mining collective would simply block XCP (and MSC) transactions altogether, making the whole protocol useless?
High chance.
What would be their motivation to block these transactions? If some of their members has assets tied to XCP or MSC, why would they want to do this?
When 0.9 is released with OP_RETURN, it will be part of the trunk/reference implementation. If someone wanted to block it, they would have to compile it out.
This argument once again brings up the point that for the health of the network, there shouldn't be so much centralisation of hash power to a few number of pools.