Try asking the same question in the the fun-fly forum.
There is a Fun-fly competition called the "Egg-Drop"...ask how they do it....

I think usually it's a plastic cup fastened to the top of the plane...to drop...just do a roll. And LOOK OUT! SPLAT!!!

There also used to be a company called Vortac that made a servo operated bomb drop mechanism.....don't know if they are still around or not.

High Flights!
CrazyHerb