Yea could be something like

Bob sends Alice's wallet's pubkey a make_invoice request

Alice's wallet generates an invoice and sends an event back with the LN invoice and a tag signaling whether or not we accept cashu and which mints we trust

Bob's wallet checks if we accept cashu and if we trust the same mint, if we do it just sends the eCash. If we don't it falls back and uses the LN invoice

My wallet after a certain threshold of eCash accumulated can sweep my funds into custodial lightning

Definitely missing something 😂