Get a signal amplifier inside connected to the GSM device inside, and an antenna outside... there are several antennas that work in L-band that can be used as GSM antenna too... and some transponder antenna. This is used in some aircraft to transmit data through a 3G
cellular network once on the ground. Voice would be disabled I would think.
The question now is, what kind of data do you want to receive/transmit? Volume? filesizes? Why not satcom like Iridium SBD
or SwiftBroadband? Equipment cost issues? Airtime cost issues?
The thing with 3G
systems for use while on air over cellular networks... would anyone want to STC the stuff if you custom make them?