ok, here you go, an externally mounted, electrically operated deployment device. The item to be launched is pushed on to the "spear" The spear pivots on the bolt that you see at the leading edge. The electrical device is a solenoid from the hatch back of a jeep Cherokee. Any trunk door opener could be made to work. As you apply power a pin pulls in releasing the spear to pivot. There is a stop that allows the pivot to just go aft of 90 degree degrees down.
In action, one minute you can see the pumpkin going along for the ride on your wing tip the next it is gone! I made this rig for the 2010 pumpkin dropping contest at Courtland 9A4. I had two tries (requires two flights and a ground crew to reload if you do not want to shut down).
On the first run I had an embarrassing case of premature pumqulation and I missed the airport! On the second run I made a respectful launch that at least came close to the target (outhouse)
The exercise was a lark but it truly gave me an appreciation for the work of the navigators and bombardiers of years past. Hitting something from the air is not as easy as one would think.