For 2 lists that I run, I use a common Filter file.  What I did is
in both lists, I have in the list header:

* .ik commonfilt

When I review the lists, the entire Filter file has been correctly
included (ie.  the header appears as if it was included normally).
However, the people listed in those addresses are still able to post.
These lists are running on Listserv 1.8c running under Solaris.

I don't have a list that uses the filters normally, so I can't tell
if it's a problem with the Filter keyword, or the fact that it's being
included.

(sample contents of commonfilt.keywords)
*  Filter= Also,
*   [log in to unmask],
*   ....