Polynesian runs, or used to run, a 733 combi 5W-ILF I think it was. Seemed strange to sit on a pallet of seats!
Most of the combis or QC's have the ability to switch roles relatively quickly. AZ used to have a 742 combi that changed it's config almost weekly. The main problem, I guess, with switching a 747F to pax would be all the running gear on the floor & PDU's etc. I suppose you could put seats on pallets, but you've got PDU controls along the walls, rickety stairs to the upper deck that would be useless if there were seats on pallets below etc, etc etc.
Remember, the 747 was a freighter by design, that was converted to a passenger carrier. And as the A380 enters service, there'll be a hell of a lot more 74F's running around.