Thanks for the suggestions, I appreciate them. I should clarify.
We have dual WAN configured, but we don't necessarily want to route all guest wifi traffic to WAN 2, because if/when WAN 2 fails over to WAN 1, we'd be where we started, as mentioned. We want to use a different IP than the IP configured for either WAN.
For example, if our primary ISP subnet is x.x.x.120/29, gateway x.x.x.121, and we have WAN 1 set to x.x.x.122, we'd like to send guest wifi traffic out on x.x.x.123.
I'm beginning to realize this will require a separate MX, because it looks like our MX100 is limited to two WAN ports (please correct me if I'm wrong there) and I can't find an acceptable way to NAT our guest wifi VLAN to x.x.x.123.
If we get an MX64 to handle guest wifi VLAN, can we leave our exiting MR physical connections in place and just static route the guest wifi VLAN to the MX64? Or would the MRs with the guest SSID need to be physically connected to the MX64?
Thanks again.