Found it: https://github.com/cashubtc/nuts/blob/main/05.md

You can "melt" a token to pay an LN invoice. To convert to Bitcoin you can plug in an LN invoice from boltz.exchange

This also makes it possible for "nutzap" wallets to do "regular zaps", by getting the invoice and then "melting" to it.

I think it's actually even possible to receive NIP-57 zaps, with a small glue layer between LNURL and the Cashu mint. The server needs to associate their Cashu mint somehow, then it give them a Lightning address. The LNURL call will make a proxy request to the "mint" endpoint internally to generate LN invoices.