I'd rather provide compilation time options for that. Variants are:

    1. ./configure --with-compressor=bzip2=/etc/alternatives/bzip2-filter,gzip=...
    2. ./configure --with-bzip2=/etc/alternatives/bzip2-filter --with-gzip=...
    3. ./configure BZIP2_PROGRAM=/etc/alternatives/bzip2-filter GZIP_PROGRAM=...

All of this is only being contemplated because of lbzip?  No one has
ever wanted "alternative" implementations of any other compressor, to my
knowledge.  Seems a painful increase of complexity.

So how about working (Sergey, I don't mean you) on making lbzip an
actual replacement for bzip?  I know that isn't being done now, but that
doesn't it couldn't be done.  Free software and all that.

Finally, does lbzip offer any advantages over xz
(http://tukaani.org/xz)?  Which already compresses better and
decompresses faster than bz2, in general.

karl