If the transaction itself pays to a certain pool's address, that might be out of band in the context that it's not calculated in the txn fee...certainly more transparent than using a credit card to pay a pool to include a txn.

I have wondered about that...if pools currently monitor the mempool for transactions that pay to their own publicly known addresses. One could pay a small txn fee and have one of the outputs be a certain pool's address, so that pool's incentive to include it is much more than other pools'.