High-wing,
The 'APU' sensor, senses nose wheel oleo extention, via the PSEU. To the best of my knowledge it is not overridable.
Nor would you want to start the APU in the air as it is not placed inside a fireproof 'container', hence you might get more than you bargained for.
The DH8 with 2 x DC Gen and 2 x AC Gen has very good electrical redundancy as it is.
I have seen instances of DH8-100s (28 seat combi pax/frt) being loaded such that without pax, the nose oleo was so extended that the APU shutdow. About a dozen guys into the flight deck compressed the oleo enough to restart the APU, but when the dozen left .....
[ 22 November 2001: Message edited by: Capt Claret ]