.... Again, thoughts?
First: I really like the fidelity bond concept, and want to see it happen.
RE: OP_RETURN : I've got a knee-jerk opposition to the OP_RETURN opcode, because it was the cause of the nastiest bug ever Bitcoin history. So I'd be more comfortable using either OP_FALSE or OP_INVALIDOPCODE for the "provably unspendable" transaction.
RE: anyone-can-spend transactions: Thinking aloud... I wonder if we might inadvertently cause "spend storms" on the network; if suddenly there are 11 BTC sitting in an anybody-can-spend txout, I could imagine EVERYBODY on the network trying to race each other to spend it (maybe assuming that there are a few miners on old versions of the software who are too dumb to claim it for themselves).