Basically your using this idea to entice people to join missions, what if, like many people say, this game is more about fighting than it is for the mission/land grab? Other than announcing on the country channel that you have a mission up there really isn't much anyone can do to "get" people to join missions. Its a fact that you'll have to live with.
Long ago I use to run missions, and join missions, but for me that isn't worth the trouble any more. Give me one good fight a night and the other 10 deaths don't mean a thing
The other thing is people have a lack of confidence in there ability to "help" in a mission. Giving them bonus perks isn't going to get them to join either. The "fear" of screwing up someone else's mission will out weigh the rewards.
Whats the answer to getting more people to join a mission.... maybe just stressing the "fun" of the mission and NOT weather you capture a base or not. If your missions start growing some of those "furball" types might start joining because you guys are just having too much fun.
After all this is a game and its about having fun, not following a set set of rules, or being held accountable for certain mission objectives, I can get that stuff at work !