I assume that you’re looking to set this comment in your Picard file naming script and not in MusicBrainz. In that case, you might want to look into the $is_complete() function. Your file naming script might look something like (untested code):
%albumartist% - [$if($is_complete(),C,P)] %album% [%musicbrainz_albumid%]/$num(%tracknumber%,2). %title%