Matt,
Yeah I agree that pull-pull can be a good way to handle the elevators but I think YOU'LL agree it's not something you'd recommend to someone unless you know they have a bit of experience. It can be finicky if you don't get everything right. What concerns me with it (and I know we have to deal with this on the rudder) is that as the plane expands / contracts, the cable tension changes. While this doesn't affect your trim, it does sometimes cause problems with the servo. I've been thinking about ways to build "balanced" systems with carbon rods so that we don't have the trim problem but I haven't come up with a good (light!) way yet. If you think about it as if it were electronics, the trim problem is seen as "common mode". We need to put it into the "differential" part of the equation.
John Pavlick
Team Black Magic, Tech-Aero Designs