> They would need to pay to the devs of that client.

No, they wouldn't. They could just find a client that doesn't require that, or build it themselves. Or modify one that exists.

You'll always get a client just the way you want. That's the beauty of open source.

> The idea is that you would know for a fact they've spent some money in that zap, at least the "tax" part.

You *NEVER*, under any circumstances, assume that you know *anything* as a matter of fact about what goes on on someone else's device, which is under their control, unless you're dealing with proprietary bullcrap. But in that case there's no point using a public domain decentralized protocol.