I'm not familiar with MPLS, but the WSA does not have anything special to accomodate MPLS setups. I do not believe it needs it. You can setup static routes for the egress traffic to use a separate interface if that is what you are asking. As long as it has layer 3 connectivity, it really does not care what the upstream does.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...