So a client sets up one or more cashu wallets on relay for the user. Cashu zaps can be received to that wallet, and this all works because cashu nuts work as backed bearer assets themselves and the control key is made the nsec. Ingenious. 

Please correct me where I'm wrong, but remaining steps to a fully integrated system are: 
1. Clients need to implement cashu wallet support (relay wallet) and support for cashu zaps (relay nuts) between said wallet(s).
2. Implement transfer of nuts in and out of the relay wallet.
3. Implement sending of relay nuts to a lightning address, such that receipt of the sats does not require the recipient to have setup a relay wallet.
4. Implement receive of relay nuts from a lightning transaction, such that the sender does not need to be aware of the wallet being on relay.

I have barely touched cashu, so for all I know most of this is already done or is unnecessary.