The wae module have two interfaces; one internal that comes when we install the module on router and the external interface. It answers one of your questions, the router without wae module there is no way to appear the internal interface.
But the wae module can only optimize traffic on one interface, could be the internal or the external, not both. So you have to configure the wccp to both routers redirect the traffic to same interface, internal or external; and have in mid that the wae module will return the traffic to one default gateway only.
Because of that, a design with the external interface connected to a new vlan, with subinterfaces on each router, routing on this new vlan, passive interface on old vlan and some kind of hsrp between the routers would work.