Yeah, certainly there isn't a good way. I've thought of another way, however. What about using the Universal Persistence Table as a way of creating these entries dynamically?
I mean, we need outgoing traffic to behave just like it was coming out from the virtual server the node belongs to. If we use the UP table (or a modifiable class) to create entries like "node address:virtual server address" whenever an incoming connection occurs and we let it there, the BIG-IP will "learn" how to do the correspondent SNAT properly.
I know that this can fail sometimes, but it seems elegant, doesn't it?
Nimbostratus