I just submitted a samba patch to add support for proxy authentication to the CUPS ‘smb’ backend. This makes printing to Windows shared printers a little bit better: instead of having to set the user name and password as part of the CUPS print queue definition, jobs requiring authentication details can now collect them from the user who submitted the job.
Meanwhile, the job is held in the queue:
It resumes processing once the authentication details are supplied. Still to do: use GNOME Keyring to store the details, to prevent having to enter them over and over again when printing several jobs.