I just don't think you would get much revenue from putting ads in-game. Game servers just don't get that high of a traffic volume. Game developers can pull it off since they get revenue from ALL of the clients that play.
The problem is, they would count by the individual connections connecting to the server, and not by how many times something was viewed. since it would be far too easy just to have people take turns spamming the Q menu or w/e. And note that is Individual connections, so the same person connecting twice would not count as 2 hits.
I may be wrong on some of this, but ads in general are a dodgy slope. They act like a gateway drug, it might start with just one ad but they seem to multiply.
Easiest and least obtrusive way I could see this working would be to include something like google ad-sense to our motd. It only appears once, everyone sees it, and its easy to add in.
I think tho, that people have to actually click on the links in the ads for it to count as a hit, but I cant remember to be honest.
Another option would be to sell Rnd merchandise.... ^.^
I bet there is a T-shirt company or somthing out there that would accept designs and sell them for you and give you a cut of the profit... That could be fun for the community. Official Rnd T-Shirt design contest anyone?