i think this could happen organically at the relay level.. for example to use the relay ( post, rdnote, emoji ), one would need to pay a sat.. this could be done basically without cost if the relay ran its own ecash mint, then users that prefer this relay could mint say 1000 ecash sat tokens with the relay that get spent when you use the relay.. top off as necessary.
Discussion
most of the custodial wallets don't charge a fee for small zaps anyway, i remember a few months ago even one of the mobile custodial wallets announced that they are changing the fee structure so the smol zap sizes are not taxed