Development of CUPS backend proxy

I am trying to write an print job interceptor. This best seems to be done in a 
backend rather then a wrapper. From the previous suggestion to chain filters 
together I though I could do the same with backends. I created a script which 
forwards all request to the parallel script. My script is called, but the print 
job is never forwarded to the "real" backend. The error message indicates that 
the file "/usr/lib/cups/backend/parallel" is busy. Look like either the file or 
the device "/dev/lp0" are locked.

If this assumption is correct :
-How can I either unlock the file or device or prevent them from being locked 
in the first place.

-Could is dis-associate my "fake" backend from the device or file to prevent 
the lock. I have to admit I don't fully understand how the association occurs.

Thanks
Alex