A lot of time has been spent discussing this on irc, it's really hard to work out a way that is in every way ideal.
While for a given set of maps, a round robin would probably give a decent ranking over the bots, it simply isn't possible with the number of bots entered (which is only expected to rise into the future), especially not with a decent sample size of maps. Janzert also makes some good points, that is that It is incredibly inefficient, while it's good to have bots play bots from all ranking regions, there is no point in having the top bots playing the bottom bots half of the time, it's just a huge waste of time, and it has the potential of ruining a bots one pairing on a map they're actually half decent on.
Completely random pairings was done for tron, and it was horrible, the top bots got very very few games with bots of a similar rank, which means the top ranked bots weren't so much decided on how good they performed against each other, I really don't think going back to that would be an improvement over what is currently being done. One option might be to start the official contest with completely random pairings, then once the bots are somewhat approaching their rightful places to turn on the better pairing algorithm for on average playing more games against closer ranked bots, but even that has issues.
The other options that I see are to incrementally freeze the bottom end of the rankings, before finally running a top x round robin on m maps, or possibly a merge sort type process, of spliting the leaderboard up into groups, with the final round being a round robin amongst each group to finalise rankings inside those groups. The biggest floor I see here is that I think rankings really need to be reset for each iteration, which means actual final rankings in your region are strictly determined from other bots inside that range.
At the end of the day, ranking is a very complicated process, what we have here is a hell of a lot better than any kind of sporting event or even a human chess championship etc. so however the rankings end, we'll just have to take them as is.
What I would like to see happen before the next contest is a whole bunch of pseudo contests run with these bots trying out different ranking methods, trying to work out which methods seem to give the most consistent rankings over the bots. A particularly good idea might be to pick out a few decently ranked bots and enter them twice, then work out how far apart they end up in the rankings. I also like the idea of using a brand new map for every game.
Edit: just noticed I didn't address your actual question, no I don't particularly like the idea of final rankings being decided based off just bots around your ranking, not to mention that doesn't help the fact that some bots are supposedly currently outside their true ranking value
