This seems like a great idea, one of the best suggestions I've seen. I can't see a reason not to do it. However, I do agree that obtaining a MiniRank should be rather difficult.
Maybe you could implement a way to obtain regular fun ranks through points as well. You know, the "Legendary" ones, "Angelic" ones, etc. Since those are obviously better than any MiniRank, those would be extremely difficult to obtain, only the most active, and/or highest donating Sythe users would have access to them.
For an idea of how difficult, say, "Angelic" would be to obtain. If 1 point is obtained per 50 posts, or per dollar donated, or per hour online, then the Angelic fun-rank would be, say, 5K points. That way, it'd take a long ass time to get. Or, you could just donate $5k, but who's going to do that?
Love the idea though, whether or not the fun-rank idea is implemented as well.