Eastern Air Lines who operated 40 L-188, had one modified for a short time to carry an APU. Unfortunately, by 1964 when I arrived in EAL Maintenance at JFK , it was long removed. It had been mounted in the rear baggage. It apparently was built in, not some slide-out arrangement although I never saw any drawings of it. Apparently, according to the old timers it produced a deafening roar in the cabin or so they said.
By the time I worked that particular aircraft, there was no sign of an APU control panel in the cockpit. However the pneumatics on that aircraft were different for the aft port ground connection was not there.
Only had the right wing root ground connection which made the aircraft somewhat unpopular with the rampies.
EAL also had a portable turbine APU on a platform which could be carried for off-line charter work. Actually never saw it used for by 1964 huffers were almost universally around. We also had a an emergency ground start hose - it had two pneumatic connectors. One was the regular one which went onto the aircraft to be started. The other connector had two pins extending out which, when plugged into another DC-8 which had its engines running, would hold open the check valve located just inside the airplane coupling. Bleed air from the aircraft running would then be routed through the emergency hose to the dead airplane.
Also, in the training department, I remember some pictures of high pressure air bottles that could be fitted. They would power, I assume, a combuster starter such as fitted to many early DC-8 and B707. EAL never had them, I think they were marked for American Flyer.