I think forwarding virtuals would work best here. Or you could use a single virtual server and select between two different pools of gateways based on the source IP address. Which LTM version are you running?
Sorry for the late reply, but just to let you know, we solved this using VLANs and Virtual Servers. We created a VS which applies for all addresses (0.0.0.0 mask 0.0.0.0) and on the VLAN we wanted to route, and we configured as pool member of that virtual server the default gateway we wanted to use for that traffic. All other traffic is routed through the default route normally.
By the way, we are running version 9.3.1 Build 37.1.