Not sure if a garmin 296 would work in a Boeing cockpit because your windscreen heating is a conductive film that may give a Faraday cage effect, giving you a poor signal. Anyone care to try?
Last count, I have about six GPS (boat, plane,car) they seem to multiply like coat hangars. The 296 works for me, but I'm only a VFR driver.
My latest Yacht autopilot has a rate gyro in it (about the size of a postage stamp - Kynetix), so I guess a hand held Garmin unit that could provide attitude information is not beyond the bounds of possibility.