See http://musicbrainz.org/edit/22301148 . The release does not have any track numbers. Not in the file names, not in the file tags. Nowehere. The edit says I successfully removed track numbers, but when I look at the release (or edit it again) right after the edit, the track numbers are back, like the edit hasn't been made at all: http://musicbrainz.org/release/646e2309-882c-4a4c-8bd5-a9a755c1a8cb

EDIT: apparently it was possible before, looking at http://musicbrainz.org/release/3cad554e-3c5a-4808-a859-1a7f03be07cc.