@wspence
I set up a similar configuration recently. Leave the default outbound NAT rule in place for mapping your main network to the primary WAN address. Select the "Hybrid Outbound NAT" mode to allow you to add custom rules to the default. Then add a custom outbound NAT mapping from the VLAN's address range to the WAN-side address you want it to go out of. Your rule should look like the automatic one except for the specific source and NAT addresses. (The automatic rules have a special case for port 500, which you might or might not feel like duplicating --- I did, but I'm not sure it will ever matter in my use.)
Don't forget that you need pass/block firewall rules in addition to the NAT mapping. In particular you probably want to block the guest VLAN from reaching your main net, with the possible exception of specific services such as DHCP and DNS.