If I'm not mistaken, Bolt12's blinded paths, when adopted widely, should replace the hold invoice approach. If Bob is provably the introduction point to a blinded path in an amount-less bolt12 offer that pays Carol, then Bob can enforce that he'll be paid when Alice pays Carol. Using this approach, it might be possible for several Bobs to be predictably paid when Carol does, assuming blinded paths can contain other blinded paths. This isn't my exact dream of a single payment being split along independent routes, to different receivers, with codependent settlement at the protocol layer, but maybe novel forms of decentralized coordination can wait for bolt13.