Forcing all jobs through a filter

[ujr at ict.pzm-luzern.ch]

How can I force all print jobs through a filter,
irrespective of the job's mime type?

I tried  cupsFilter: "*/* 0 filter"
in the driver's PPD file but the filter is not
invoked when the print job is created by a PCL
driver (from Windows through Samba). It works
when the PS driver is used on the Windows side
or if a job is sent (from Linux) directly to
CUPS. Any insight on this is appreciated.