Alaska Airlines had expressed interest in a combi version of the -700, with a fixed configuration of 3 palets and 56 seats, similar to the 3/56 arrangement of their 737-200QCs. But with the stringent FAA regs regarding freight and pax on same deck (excpet for the grandfathered a/c, like AS -200QCs), floor strength and fixed bulkheads I dont think the idea went any further. Additionally I dont think Alaska would have much use for a fixed config like this since they reconfig thier 737-200s up to 3 times a day to meet the demands of the particular routes.
It would seem like a costly venture to order new 737-700Fs when a freighter conversion of a 737-200 or -300 would be MUCH cheaper when you get an a/c of the same size/ capability in the end. I guess unless you need the extended range of a -700.