Great idea...question about standards/technicals when signing a message proving knowledge of a pubkey associated with a bitcoin address possessing UTXOs of a certain value...

For single sig, it's easy. One privkey-->one pubkey-->one address. Verify the UTXOs on the ledger, here's a signed message corresponding to its pubkey, done.

For multisig, it's more complex. I can give you an address and you can verify the UTXOs, that's easy. But I'd need to give you all of the pubkeys that combine to make the address, so you can calculate it yourself, and then sign however many messages with those individual it would take to reach the spending threshold.

It'd take more effort to create, and more effort to verify. I wonder if there are already standards that exist for this, or if there are any in the works.