RE: Long lasting gas powered Sailplane
So what you are really talking about in fact is a gas (glow or heavy fuel) engine powered system, fitted with a generator that can recharge the onboard batteries of the avionics. This is standard practice for UAV's.
The link you provided for the electric system still uses a gas engine for power. This will not let you fly an electric powered aircraft forever. You may be able to extend run time slightly, sort of like regenerative braking on a hybrid car, unless of course you want to ignore the laws of physics. There is no way to get free energy in a closed loop system. Thats called perpetual motion, and it dosen't exist.