Unfortunately due to some poor design decisions made before I arrived, certain servers need to go out via certain virtual IPs, so manual outbound NAT is a requirement in my case.
@phil.davis:
As well as having NAT rules to apply NAT on the way out to the public internet for packets with private IPs, you need firewall rules with the gateway specified to direct particular stuff to particular WANs.
That completely answered my question, thank you very much for the help!