You're right, there is a place for a 120mm fan there. However, there isn't enough clearance around the hole for the radiator (the recessed portion for the expansion slots interferes).
A completely external solution like yours should decreases temps in the case a little bit too, so that's always a bonus
Maybe I can do both the pump and the radiator outside the case. I'll find shoebox or something and turn that into my ghetto external cooling system