NetBSD-5.0.2/sbin/mount_portal/examples/rfilter.1

If you don't already have bzip2 and bzcat installed, this example
won't work.

Use the following configuration file (also in
/usr/share/examples/mount_portal/rfilter.1.conf):

bzcat/	rfilter	bzcat/	bzcat %s

Mount this configuration file, with a command like:
% mkdir portal
% mount_portal `pwd`/myconf `pwd`/portal

Now create a bzip2 file, and test whether it all works:
% printf "This is\na test\nfile\n" >/tmp/bziptest
% bzip2 /tmp/bziptest
% cat portal/bzcat/tmp/bziptest.bz2