Yeah a sheepskin or one of those beaded things would be good for the long rides.
The F650 is a great bike for Thailand, especially as I need it to commute in BKK everyday as well. I feel anything bigger wold be a pain for me, most of the time.
There only a few niggles which make it uncomfortable on long rides which are the seat, too low a screen and a heavy accelerator. The first two can be fixed easy but not sure about the third.