zutilsrc just allows changing the command used to decompress each supported
format, but it can't be used to define new formats.
I don't want to add any new compressors. User will. And it's their responsibility to do it right. Not yours.
Adding a new format multiplies the troubles because every decompressor has
its own quirks, like undocumented exit status or skippable frames.
What you ask requires the redesign of zutils. Currently adding a new format
requires making changes to most source files.
It's not necessary. I ask to add abitlity to parse extension and then use program defined in configuration (file). For example
-- cut zutils.conf --
liz = /bin/lizard
--
$ zcat abc.liz
Would run /bin/lizard [1] and decompressed abc.liz with it into stdout. You don't have to care about frames, skippable or not, it's decompressor's job. You don't check whether archive.liz is correct or not, lizard does.
Without it in configuration it would just cat the file.
It's like with the tar:
$ tar c -I/bin/lizard -f archive.tar.liz files...
$ tar x -I/bin/lizard -f archive.tar.liz
And it works. Tar won't know what that actually does, except it de/compresses to standard output.
[1] https://github.com/inikep/lizard