There is the cashu-nut standard for nostr where multiple wallets can share ecash tokens over a private (nut-to-self) nostr message. This enables multiple wallet implementations to share a single balance and effectively replicate the utility provided by multiple frontends (like lndhub or NWC protocols) to the same lightning node or account.
Discussion
Yes, but there should be competing cashu wallets and I should be able to use whichever I choose wherever I choose.
There are at least a half dozen competing cashu wallets. Not sure what you mean. Should they all support nostr-backend? Probably yes, but it's a relatively new idea.
I mean that apps shouldn't hardcode one in.
What I think is missing right now is the ability to send and recieve cashu tokens over NWC. You should be able to zap cashu tokens directly and when you recieve them, your nostr client should transfer them to your NWC connected cashu wallet without needing to go over lightning.