Using F5 as default gateway in Amazon AWS

This appears to be the classic problem of getting the return traffic back through the LTM, which isn't specific to AWS. The problem you are facing then, is that you can't put a more specific route on the pool members because you don't know the source addresses clients will come from (ie: Internet).

If this is the case, the simple solution is to apply SNAT Automap on the Virtual Server. Among other things this will allow you to retain AWS as the Default Gateway on your Pool Members. The traffic to the pool member will then be sourced from the BIG-IP's Self IP on that internal vlan.