Paralelní Polis in Prague runs BTCPay and they have some tipping functionality, though I no idea where that actually goes.
I'm hoping:
a) phoenixd takes off and they add a plugin.
b) btcpayserver (npub155m…dcvg) releases this mysterious app they announced last fall
The combination should be a acceptably self-custodial KYC-free easy to use portable lighting accepting thing. And from there merchants can continue down the rabbit hole.