On my Passat (and all newer VW/Audi automatic transmissions), you had to push on the brake pedal to shift from Neutral to Drive, but you didn't have to push very hard, just depress the pedal half an inch and the gear selector lock would disengage.
But I dont think thats the problem. The DSG is still an automatic, so this question of "Push starting a DSG" is like asking can you Push start an automatic transmission? That's simply not possible.... or in other words "Not easy".
"Most vehicles with automatic transmissions cannot be started this way because the hydraulic torque converter in the transmission will not allow the engine to be driven by the wheels (some very old automatic transmissions, e.g., General Motors' two-speed Powerglide transmission, do leave a solid connection between the engine and wheels, and cars equipped with such transmissions can be push started)."