It's a bit different than this scheme, but I actually read a short story in Dragon magazine about 35 years ago (very likely 1980 or 1981) about an MMO where most people paid a subscription fee to play, but you could exchange in game currency for play time (and could even cash out into real currency -- leading to pro players who would compete for collecting the most in game gold). It is an interesting twist to have the players front the real world cash.

As an aside, I always dreamed of playing the game I read about. One of the cool things about the game was that your character continued in the game as an NPC when you were offline. I always thought this would be very interesting way to have player generated content, where they would have to build defences to protect their player/gold while offline. Other players could launch raids to try to steal it. No one has ever built a similar kind of PK system that doesn't piss everyone off, but for me it's a kind of holy grail of MMO functionality. I hope I see it before I die :-)