How would this work? If I send ecash from some random mint to btcpay server instance it would still need to make a lightning withdraw from the mint to finalize payment right? So why not send via the mints lightning in the first place?
Discussion
You can send Cashu without Internet
Conceivably, by smoke signal. Or carrier pigeon.
No. There is no need to make a Lightning transaction until the sats leave the mint. A merchant using BTCPayServer would receive eCash (issued by MiniBits mint, for instance) and it would remain as eCash until they decide they want to redeem it to Lightning. There is nothing requiring them to redeem it upon receipt.