Maybe dynamic routing protocols can achieve your requirement....
CP15600 cluster points the default static route to internet router, then redistribute to ospf instance, then core switch would learn this default information, the client traffic will then go through core switch->CP15600 cluster->internet router, as for the return traffic, because internet router will learn all the vlans information from core switch, so return traffic would be internet routers->core switch->user subnets.
Or you may simply use PBR on internet routers to force return traffic go through core switches.