What material to use for non metalic pull-pull setup?
Kevlar thread. Not sure where to get it but that's going to be low stretch. For less demading applications you could probably get away with 1/2A Dacron controline thread. It's low stretch and strong compared to most stuff but the Kevlar would have it all beat hollow.
I think they even use Kevlar in place of steel cable for larger controline models now.