It seems that the current most prohibitive figure is the number of people with activity on other SE sites. Not sure if advertisement on SO is allowed, but one sure method would be for people to get an account with SO and ask/answer one or two questions. Its not very hard to get above the 200+ rep.
I think it would be a good idea to just go and tell as many people as you can find. If you know anyone who is interested in Robotics, e-mail them, ask them to commit, and also introduce them to some of the other Stackexchanges that they might find interesting.
Mention this on other forums. Mention it in the chat rooms of other Stackexchanges.
We're actually doing very well. We're about to pass Theoretical and Applied Robotics (26%) and we have more committers in total. And we've done this in just 2 days!
Another problem is that only +- 25% of the person who commited here are active on stackoverflow, and even fewer to another sites. Asking questions (relevant ones, of course) in other sites would help a lot.
I would not worry too much for now about that particular issue, since the Robotics proposal does not have the necessary number of committed people anyway. We have currently a ratio of 1/4 between the total number of committed people and the number of committed people with 200+ reputation