ECMP + PPTP = Frustration

Anyone run a PPTP server and lock it into just one interface and still have ECMP for all other traffic?

If I try and use Routing Marks its seems I mine as well disable ECMP because I end up forcing anything in WAN1 to go out WAN1 which is not what I want.

I wish I could tell the PPTP server to exist on 1 interface, since load balancing will break the connection, but it seems I will need to add packet marks, routing marks and connection marks is some strange judo move to get this to work.