It seems that this might be most useful if implemented similarly to mkfs or fsck, in util-linux. For instance, if a file is named foo.gz, zcat would use zcat.gz or gzcat if available, before falling back on its built-in decompressor.
That way, if someone makes "thetazip" in the future, they could provide zcat.?z in their package; zutils would support the new format without needing to be updated. Also, maintenance would be in the hands of the developers maintaining the package; they would be best able to keep up with changes in their format.
Just a thought.