Hi Justin,
It sounds like you want to load balance the requests but not have it handle the outgoing traffic. There is a technique called npath routing which sounds like it's what you need w/o putting in an iRule.
The nPath method of traffic management increases outbound throughput because packets do not need to be transmitted to the BIG-IP system for translation and forwarding to the next hop.
Check it out.
http://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip9_2solguide/BIG_IP9_2SolutionsGuide-03-1.html?sr=10627741
I hope this helps
Bhattman