Hi Dave,
Didn't think to suggest you check what the Windows users had installed. Well done.
I just finished confirming the workaround in my lab to an iRC4080 and it looks a winner.
There is two parts to the solution.
The first is that you need to enable a setting called 'Allow Unknown IDs' located under Dept ID. Hopefully this option exists on your model. The setting allows print jobs without an ID/pwd to be printed. The setting is located under Additional Functions > System Settings > Department ID.
The second part is that you have to use a non-Canon driver, so that the Dept ID setting is not enabled in the driver. If the machine only had UFR2 support, we would have no options. But thankfully the machine does have PCL5e support, which means you can use the Gutenprint Generic PCL5e driver, that should have been pre-installed on your Mac.
You will need to create a new printer queue and you can probably delete the current queue that would be using the UFR2 driver. Before you do, make a note of the IP address of the printer (or find that Windows test print which will also show the address).
For the new queue, select IP > HP Jetdirect-Socket (this is what the Windows users will be configured to use). Enter the IP address of the printer, no queue name is needed, give the printer queue a name and then open the Print Using menu. Click 'Select a driver to use' and then in the search window that appears, type Generic. This will short-list all the Generic Gutenprint offerings. Select the PCL5e. Click Add to finish and then try printing something. You should find that no pop-up appears prompting for an ID/pwd and, more importantly, the document prints on the machine.
Let me know how it all goes.
PaHu