out of band fees can work in both directions, e.g. including rebates. (and, in fact rebates can be done inband with coinjoins with no trust)
Also consider what your scheme does when a majority hashpower censors any transaction paying a high inband fee level.
I'm not sure I understand how you're suggesting a rebate would work?
What do you mean by a majority hashpower, a 51% attack? If a transaction has a high fee surely any miner is incentivised to include it in a block?