In any case, let me propose actual improvements to the OP_BRIBEVERIFY proposal:

1.  Remove the necessity of coinbase commitments.  The miner commits to the sidechain_id and h* in the transaction that pays the OP_BRIBEVERIFY anyway.  That way the h* commitment occurs only once in the block, in the transaction that does the OP_BRIBEVERIFY.  In addition, there is no need to impose particular ordering on the coinbase outputs, which would be problematic as pointed out by others, for example if the miner is interested only in merge mining for sidechain id #35 and nobody else.

2.  When verifying a block, keep a set of sidechain ID's.  When processing a transaction in that block with OP_BRIBEVERIFY, check if the sidechain ID is in that set.  If not in that set, add it to that set and continue script processing.  If already in the set, fail the script processing.  This ensures that at most one OP_BRIBEVERIFY exists for each sidechain_id in a mainchain block.