People can obviously commit to such bounties now, having received all the tips, but it'd be nice to have it enforced.

The difficult case is what to do if no one contributes a comment that's selected. They could

1. sacrifice the tips to SN to be redistributed
2. or it gets evenly distributed to all the commenters (although that incentivizes poor content)

The other trickiness is the UX of choosing and communicating this contract ... perhaps though it could be the default on any Ask SN.