I like this idea because I've seen similar for the likes of Minecraft bukkit servers with their player-driven economy. Just tie it in per server, which I think you said, then that's fine. That, and NPCs getting involved.
Reading what Arcane is saying about EULA, Mojang, Notch, & even blacklisting is the fault that should stay over in the Miencraft world. People have been shitty over there that they should stay over there. What I'm seeing here is fine, and if it works harmlessly for its own digital currency to tie into NPC traders & such then that's fine. I see no issues with it. You're trying to tie servers, yet that may get highly messy in itself that I'd just say keep it simple to have it be server per server thing, unless people start making multiple worlds to behave as one.
Seeing people put those weird packages of paying with their parents money to buy stuff to support a Minecraft server is pathetic, and why I preferred privates servers. Still do. Glad what is being suggested is a free thought & concept. Mojang community is toxic and shitty, with special thanks to spoiled brats of a hoard of kids with shitty parents. Kids assuming they're all special so they try shouting back showing how spoiled they are by using their parents cards for being ripped off with in-game items. The same type of items you can just get in the game, yet you need to pay to get them, or to be privileged into getting them. No need for that. Everything should be at your fingertips with no pay-gate. I find the blacklist proper. But that's really not part of this topic.
Arcane having a rough day, or just trying to stir up drama. Hope it's the former because there's plenty of rough days out there.