I've also tried to accomplish this and I'm fairly certain it isn't possible. The NAT will only be for external traffic coming in. Everything going out will go through the MX WAN IP. Depending on your use case, the only real option would be to put an L2 switch outside of your MX WAN interface. Have one cable going to the MX WAN interface and another going to your switch VLAN or device and then you could give those devices the WAN IP directly although they will not be going through the MX. I guess conceptually if it is for a guest network you could also have a separate, cheap, router that is connected to the WAN that you route traffic to/through.
Adam R MS | CISSP, CISM, VCP, MCITP, CCNP, ITILv3, CMNO
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