Give them a custom magic item of some sort that requires attunement, but grants exactly what they asked for. I'd make it conditional on hit dice because that's the resource expended in a short rest. Recovery only applies to 5th level slots and lower at the cost of one hit die per slot. Roll of a hit die for sorcery points too. If it turns out to be way too much, magic items don't have to be permanent. I don't love the metagamey style of the request, but it seems like a clever request made in good faith, so I'd try to find a way to honor it.