Rob,
Your concerns are well founded. Soldering on push rods can damage plastic bell cranks and horns. I prefer to use wheel collars and JB Weld epoxy. In the attached photo, even though I did not have any plastic to be concerned with, my preference is still for the wheel collar and JB Weld.
A couple of details: I like to file/grind a GROOVE for the set screw, so that in the very remote chance that it comes loose, it will not be able to slide down the wire. Also, put RED Loctite on the collar set screw. To maximize adherence of the JB Weld, degrease all surfaces that will be epoxied, and also roughen all surfaces with coarse sandpaper to provide a better surface for the epoxy to grip.
Take your time and do a through job here, as the control system is one place when no failures are allowed.
The plane in the photo happens to be a 40 size Time Machine, and the flap horn, with bushing, is from Tom Morris.
Cheers.
Warren Wagner