Hi Shawn,
The SERVER_CONNECTED event is too late to change the SNAT configuration as the serverside connection has already been established. It should work if you change to LB_SELECTED which is triggered when a load balancing decision has been made. You would also want to use LB::server to get the IP address of the selected destination host instead of IP::server_addr.
[Edit: as Denny pointed out, if this is a VIP with address translation disabled (like a forwarding VIP), you could use CLIENT_ACCEPTED and IP::local_addr to check the destination IP address.]
That said, you could probably define two separate virtual servers--one enabled only on the external VLAN for external clients and one on the internal VLAN only for internal clients. You could then enable SNAT only on the one virtual server.
Aaron