Hi Frank!
Instead of adding overhead to rtpproxy to deal with this how about setting up a bonding interface/trunk interface/... on the underlying OS?
In a nutshell you aggregate multiple ethernet interfaces into one virtual interface. The virtual interface gets one IP. As part of the interface config you can define the load distribution mechanism (everything from round robin, failover to more complex link aggregation protocols).
Big advantage to me seems to be that you can change/upgrade the underlying infrastructure without having to fiddle with your rtpproxy config/source.
For FreeBSD check the if_lagg or ng_ether modules depending on the version you use, i.e. http://www.cyberciti.biz/faq/freebsd-network-link-aggregation-trunking/
I hope this helps, Hendrik