Just a follow up. I couldn't find anything about Pentax being used on US drones, but
Controp, a company providing imaging options for Israeli drones, uses Pentax. Israel has close links to the US military so it's not a stretch to guess it's the same stuff used in the US. This is it:
The MGF1 is based upon a Pentax 645 body and lens with a purpose-built large-capacity 56 × 41.5 mm (2.2 × 1.6 in) film magazine, including an Eastman Kodak S-149 supply spool containing 76.2 m (250 ft) (which will provide 1,600 exposures) or 45.7 m (150 ft), which will provide 1,000 exposures. Each exposure has six digits of annotated data per frame providing information upon the date, time and frame count. Lens options range from 35 mm f/3.5 to 600 mm f/5.6. The sensor provides manual or automatic exposure control, exposure modes being aperture or shutter priority, programmed AE or manual. The cycle interval is a maximum of 2 seconds per exposure cycle and the film resolution is better than 70 lp/mm. The camera is powered by six 1.5 or 9 V AA batteries.
For what it's worth, whilst poking around Jane's, I also found mention of Pentax being used on AUVs - underwater drones.