I am fairly certain that there's nothing wrong with accepting donations, even if you compensate said donators, so long as the money does not primarily go towards personal use.
Spend it to upgrade servers, pay for server hosting, maybe even pay a little for your own coding or other members of the team (NOT MEGAUPLOAD LEVEL, less than you'd earn at a gas station..) and I think you're in the clear.
You have to be a pretty big company in order to sue and win over "donations," and the infractions have to be pretty obvious (that dude who was earning, and pocketing $1 million/year off his WoW free server by flat out giving people whatever they wanted for $5/$10 here and there, and continued after repeated warnings comes to mind)
But the first step, I think, would be to try to contact the copyright owners of M&B and ask their thoughts/blessings to do so. I for one would not mind larger servers ONE BIT. I have personally witnessed the strategus servers being nearly completely full, with a lot of spectators complaining about not being accepted, and also randomly down. Would be nice to have larger battles and more server stability.
Whether or not I agree with the donation bonus or not, I'm unsure.