Is the information that TuneUp writes to tracks visible to other software or devices (on my home network)?

TuneUp uses the Apple-published programming interface to write the information it finds into your library. Then iTunes takes that new information, and appends it to the file's ID3 tags (or equivalent).

Since there are various versions of ID3 tags (v1, V2.X, etc.) and different ways of saving the information in the tag itself (reverse unicode which affects Korean/Japanese/Chinese/etc.), you may have to use the "Convert ID3 tags" feature within (PC) iTunes to get all the tags to be visible on some devices or software.