Hello, on a multi-wan router like an RB750G can a rule be constructed that says...

if traffic comes in on a particular interface then it should also return via that interface, regardless of what the default route is?

Regards,
Maxster

Yes, look at the PCC manual on the wiki - that's (in part) exactly what it does. Just add rules to do the same thing in the 'forward' chain so not only to-router but also through-router traffic is handled that way.

http://wiki.mikrotik.com/wiki/Manual:PCC#Policy_routing