…issues have been fixed with the most recent printer driver update for OS X Lion. Network computers can print A3 to the shared inkjet. Both the host as well as the network computer needs to have the up to date software. For the client computers, you need to go to System Preferences and delete & re-create the printer after the update.
You have the working driver (as of the moment of writing this) if your System Preferences says you have Driver Version 10.51.2.0, on both the server as well as the client computer.