Have you looked into any 200cc or 250cc single enduros?
I know Charlie from Parabellum Fairings used a NX250 for one of his mileage cycles. Not sure how many are liquid cooled - maybe the KLX250.
NX250 -
http://www.bikez.com/motorcycles/honda_nx_250_1989.php
Looking at the latest KLX 250 (
2011 Kawasaki KLX 250 specifications and pictures), it sounds like a decent canidate - FI, 6-speed, liquid cooled. Not sure when it was first produced like that though, so it may have to be pretty new.
The little Ninjas are cheap and available, but overall a FI single would still have significant mileage advantages I would think. The little Ninjas are pretty weak down low where you'd want to be cruising if you were to try to gear it higher.
Jay