Hello Tom,
I don't think that this is possible: the router is designed to forward / route traffic not to take part in application flows.
You would need to deploy an host on the other end to send traffic in the opposite direction.
By the way, this is usually the deployment scenario for tools like netperf or iperf.
see
http://www.netperf.org/netperf/
http://iperf.sourceforge.net/
Hope to help
Giuseppe