Yep, that's basically what I'm working on right now, how far away from the Primary I want the Support to stop.
The issue is getting the Support to stop behind the Primary no matter what direction the Primary is going. TM has a really cool sub but it was part of a pretty super elite project and he'd prefer it not get out. I posted up my idea in another thread and I'll be trying that solution whenever I get some time to work on it.
I think you'll like the Swap Bot even better then this. EN has a couple very simple subs that makes swapping clients very easy, and it's not restricted to number of clients. You just tell it who you want to swap to and it does it.