Yeah sorry, I can't think of anything to solve that. But it does have me thinking that I bet someone out there has made a wrapper program that intercepts networ

Yeah sorry, I can't think of anything to solve that. But it does have me thinking that I bet someone out there has made a wrapper program that intercepts network system calls (using LD_PRELOAD hacks) from a target executable and routes them all through a specific interface. Failing that, you're probably stuck with iptables and maybe Linux namespaces.