That's not what I meant. You use ecash but there's a gateway. You go to a vendor and pay with LN through the gateway. The vendor might receive it in ecash through the mint he trusts or can just use LN directly, you don't care.

This works in cashu/fedi already. The gateway doesn't have to know who you are. The part where you give up privacy is the chat: your Fedi client connects to the chat server(s) asking "any dms for nullcount?" when you open your wallet.