I'm not sure how many tree co.'s would want to hand the climbing position over to a sub-contractor. The climber is the boss, the position with all the risk and all the glory, the central element that the entire rest of the job revolves around. Many tree guys have a bit of an ego and fancy themselves as being pretty bad-azz. I don't see them trusting that or wanting that to go to a sub.
I've known tree companies large enough to have their own crane who used to rent them out to other tree companies but have since stopped doing that. They said that they were getting subbed by too many idiot tree co's who had no idea what they were doing, how to climb or how to work with a crane, and yet were still insisting that they were the boss and they were going to climb and "we're only paying you for the crane so shut up and climb back in the cab!" I realize this is the exact situation you're considering filling, and while I think it's a great idea, I'm not sure if it's a great business idea. How many companies would be smart enough to employ your services? Are you friendly enough with any of the local tree co's to inquire about whether they would consider using a climb/crane sub?
Just my $0.02. Well, $0.01 is more like it.