Using scripting, cyou could add it to the files as your own custom NTFS ADS metadata, assuming the files are on a local NTFS drive and won't be archived or copied to something that isn't NTFS (e.g. cloud storage, NAS). Only your own script would know to look for it, so it wouldn't show up in other tools.
It could also be added as generic tags, but you'd have to put the whole key and value into the tag name, which might be messy but could also work in an easier way, depending on what you need the values for.
Or store them in a separate database wherever you want, using scripting.