I think the greatest cooling factor is air rushing through the armature of the motor. Restricting this in any manner sounds like a bad idea to me. Correct me if I'm wrong but your idea is to attach liquid cooling heatsink to the router casing? If so I don't think this would make a significant difference but I'm interesting to hear what others have to say.
Not sure what you are cutting or your build either. Wouldn't hurt to know the specs you are dealing with.