Github tag/release for subrepo

Question

I was pushing recent changes of my repository to Github with a tag, and then published a first release. But I need to create a tag and release only for one of my subrepos, because it is a self-contained software, while other subrepos are related to this one only historically (parts of one project). At the same time, I can't restructure this repo. As far as I understand, any release will contain all the subrepos and, what's more important, readme.md from root, not from the desired subrepo. What would be the best practice to create such release?

Tried: a few pushes from my local repository and subrepository to remote with tags and checking the content of .zip files.