The British Skydiving Operations Manual Section 8 (Skydiving Limitations) Para 5 (MAXIMUM ALTITUDE) Sub-Para 5.1 (Skydivers) states,
5.1.2. Skydiving will only take place between Flight Level 120 and 150 without oxygen, provided that:
a. Skydivers are not above FL100 for more than 30minutes before leaving the aircraft.
b. Skydivers are not above FL120 for more than 6 minutes before leaving the aircraft.
c. The aircraft commander monitors these flight levels and time limits.
d. If the flight level or time limits are exceeded, the drop will be aborted, and all skydivers will land with the aircraft.
Whilst it is the PIC’s responsibility, I think it would be a very useful feature to have an alarm that warns when these conditions have been exceeded. It would also require implementing of Flight Levels if they haven’t been already.
Before anyone says it, blah blah, nanny state. 
Thanks,
Craig
