This forum is focused on consumer level products. For the Designjet you may have better results posting in the HP Designjet forum here. I do not know if anyone there will have Linux expertise but they certainly have a number of Designjet experts.
You might also look in the HP Linux Imaging and Printing Site, while the Designjet T120 is not listed there are dozens of supported Designjet models, perhaps one of them would be suitable for your model.