Its been done before with great results. I'm trying to find the forum now where I had seen this done.
I think the person who did it was able to get a good 10 miles of video / control signal.
But then again. Your in control of two birds at one time, one flying the repeater, and the bird your flying the distance, and if you need to use a repeater it means your way out of LOS, which is a big no no.
Its a great idea, but adds a lot of complications in more then one way.