Pyro himself has already replied to a thread about towed artillery. He regrets that it isn't in-game and wishes there were an easier way of implementing it, but it isn't all that feasible with the current game engine.
A couple of ideas to make it work in the current system: -
They could just make a half track that when you press 'O', the gun moves away from the vehicle and can be manned using the '2' key. It isn't truely hitching/unhitching, but it would make AT guns possible. This way, in the coding both the half track and the gun would only be one vehicle with the gun set as a gunner position.
Alternatively, the AT gun could be set to be similar to a bomber drone, so that it follows the half track, and when the 'O' key is pressed, rather than follow the half track it stays in position and can then be manned using the 'ctrl 2' key.