Short answer:
You can't get the metadata for tracks beyond the MB API limit currently with Mp3tag and the available Tag Sources Scripts.
Longer answer:
There is an API limit from MusicBrainz. You can read about it here.
The only thing that would work, is a so called "Pseudo release". This kind of a splitted release would have to be added at MusicBrainz first.
Currently, you can't limit the amount of tracks in Tag Sources Scripts, like tracks number 0-500, then 501-999, then 1000-1499 and so on. This would be necessary to get the tracks beyond the API limit.
The only "workaround" would be to try Picard, the official MusicBrainz tagging software.
AFAIK, such kind of paging was introduced in one of the latest builds (v2.8):
