I want to assign a custom tag for files in a project folder to use the tag for sorting and grouping in Windows Explorer.

There are three limitations:

1. No system modifications (== no shell extensions).
2. The tag should be assignable to any file type (and folders).
3. The tag should be clonable via git.

The set of possible values for the tag is rarely changed.

So far, my best approach is creating pseudo user accounts (with limited rights) as the tag values, sharing the files with them and using Shared with field as the tag for sorting/grouping.

But Shared with is a local property and is not clonable via git.