Problem with servers using F5 as DG

I may still be missing some details, but I don't think you have a lot of options without SNAT. Default gateway or not, even if the packets from a server made to a pool member, it'd carry a client source address that the pool member would know how to route back to directly. Besides, a server wouldn't use its default gateway setting if the source was on the same subnet.

I'm beginning to believe that accessing a VIP on another BIG-IP VLAN, using the BIG-IP as the DG, is another problem entirely. I can't say for certain if that's supposed to work. In any case, if you recreate the VIP on the internal VLAN for internal servers to use, you shouldn't need the DG. The only alternative to a SNAT, I believe, is static routes - which would get messy.