To get energy to the electronics in the plane, you can use a pair of small propeller blades attached to a pair of dinamos, that will recharge a small battery, so you will use the dinamos as a way to reduce the amount of kinetic energy, and power the electronics and, possibly, actuators.
Excess energy to heat the electronics enclosure.
As a computer control, you can use an HTC Dream, with android, that has accelerometer, magnetic field sensor, wifi, gps, compass, video camera, is fully hackable, and you have the source code. Should be relatively easy to hack a usb device to control the surfaces of the plane, using PWM and electromagnets. Log all data to the 8Gb microSD card, that propably will survive a fatal crash, flushing the filesystem every 1s. The program for controlling the surfaces don't need to be too sophisticated, as will be using closed loop sevos with the set points controlled by the GPS. The accelerometer can be used as the D paramenter in a PID algorithm.
Stability of the system can be easy to verify with a couple of equations, if needed, but much more fun to launch the plane from the roof.
In addition to the GPS and accelerometer on the phone, you can use a small balloon, that will activate a switch when the outside air pressure drops enough.
Use an electromagnet inside the plane, attached to a piece of iron hanging from the balloon, that will be unpowered on command from the phone and will use the mechanical altitude sensor as a failsafe (serial installation). D Cell battery powered.
Recommended design flow:
1 - Specify hardware needed for the system.
( Just draw a diagram with names. )
2 - Calculate hardware weigth.
( Group all the parts, and put them into a kitchen scale. )
3 - Calculate wing span for plane + 2 x hardware weigth.
4 - Model airplane in X-Plane simulator, while the computer hack is preparing the programs, and the HW hack is preparing the rest of the hardware.
5 - Evaluate total payload weigth, and size balloon acordingly.
6 - Build aircraft.
7 - Test fly it.
8 - Remove possible problems.
9 - Get BAA permissions.
10 - Send the plane.
My 2 cents.