Uniquely identify user (by PPD?)

[Gary Carroll]

Consider a campus environment where users supply their own computers, 
which may be of differing operating systems. The users may print to a 
CUPS print server on the network and retrieve their job by swiping their 
ID card and selecting the job from the list.  
The trick is that I want only their jobs show in the list, and I want to 
limit the custom software installed on the user computer to a PPD. 
I can see a good deal of information about the incoming job from the 
network, but this is not always enough to guarantee that I can identify 
the specific user. For instance, while I can identify a specific 
computer (by mac address), some computers may be shared, and there is no 
validation that login names are unique.
I would like to have a user have the option of entering a user ID in the 
print dialog box and have this passed to the print queue (as a comment). 
However, it seems that (a) a PPD only allows me to select from 
previously defined choices, not input a string, and (b) using edited 
PPDs to customize dialog boxes seems depreciated if I correctly 
interpret my scanty Apple documentation.

Does anyone have a suggestion, or pointer in a promising direction?