A possible work-around for the original request could be to make any desired filters to the notes in Paratext first, then from the Notes list window select Edit: Copy all notes and paste into a text editor and perform the regex search there.
Alternatively depending on the need, in Paratext one could search for the certain word (e.g. UNS) and then assign the notes that result from that search to a custom category (probably manually). Once that is done, a custom filter could then be used to exclude that new custom category of notes. If UNS is uniquely used for e.g. comments by an uninitiated speaker, that would be a good use case for custom note categories.