One issue I'm running into when implementing this is that you need access to the locked privkey to redeem the ecash.
If the ecash is locked to a key that you encrypt with NIP-44 and store on nostr you run into the issue of many (most?) extensions not implementing NIP-44. If the ecash is locked to your nostr key then you need that privkey.
This kinda sucks for web clients that don't allow nsec login unless I'm missing something.