Old MODPlug player and also Tracker 1.16 has the ability to open a file from a zip package.
Either if that song is in the .zip format or renamed to .ITZ, etc. That is nice for samples because you get a form of compression.
OpenMPT can still load these songs if its just one song in a zip file. Wouldn't it be great if you can also save back into this format?
There's no quality loss as in MO3 because zip compression isn't lossless. True, the compression ratio is not as big as MO3, but all the tools are there already.
Maybe implement 7-zip compression for even better compression. 7-zip is open source.
Open Source doesn't imply that the code is compatible with our license. We don't actually know where the zip/rar/lha code comes from so it's dubious anyway and should be replaced at some point - Adding zipping support would be a nice thing to have then as well.