I have just been wondering if there would be any sense in combining the
CEV, (whatever that may be), with an upper stage propulsion system.

This might give you a flexible and reusable upper stage and perhaps save
some weight over a separate system approach. In addition to negating
the need for a separate upper stage, this might enable a larger
effective CEV on smaller effective launchers. It might also entail a
fluffier effective capsule design, easing reentry requirements.

With refueling this might also directly open up the earth-moon system,
though it is a one size fits all approach.

Would there be any useful advantage to such an approach? Is the added
complexity prohibitive?

Pete.