Does any pool accept higher-fee transactions of a double spend, instead of the earlier one? Some pools use a modified bitcoind that uses different rules for choosing the transactions to include in a block. Are there any pools that knowing one transaction to be included in a block would discard it if another transaction on the same inputs would arrive with a higher fee? That is to say, when faced with a double spend attack between a transaction that arrived earlier and a transaction that pays them more, the pool would choose the latter?
It seems as if there is such a pool now, advertising the behavior as a feature to allow "reverting mistaken payments":

BitUndo

They also supply a patch to bitcoind.