Have you looked into NIP-60 yet? Your idea sounds more general purpose, but we are working on a cashu-specific one: https://github.com/nostr-protocol/nips/pull/1369
Discussion
Yep. NIP-60 was my starting point, but I realized I could do more general stuff. My primary motivation is to swap out an internal custodial wallet component I built that relies on a Postgres database. I realized I could use a relay, and with better security (removing my database as a honeypot of data). I implemented the proof storage as per the NIP and added in some lightning capabilities.