I have a CD that has been downloaded and completely tagged, but it doesn’t appear in Discogs or Musicbrainz. I would like to be a good citizen and upload the information to these databases. Is there any method to do this automatically or semi-automatically with MP3Tag, or must it be done by copy and paste through the web interfaces?
There is no method, you have to use Musicbrainz's functions.
Those metadata databases provide a dedicated process to enter data via a web interface (cf. MusicBrainz Beginners Guide, Discogs Submission Guidelines). Once submitted, the data also becomes available in Mp3tag through one of the Tag Sources.