This is possible now with the _FORMAT field like @ohrenkino suggested. But it's complicated if you are not comfortable with deeply nested expressions and regular expressions.
However since the _FORMAT field, even when containing multiple tag-value pairs, is still only one field, it does not help for multi-selections as only the entire field content is compared, not each tag-value it consists of.
Here's a video to show you exactly what I achieved going off of @7stones's idea and where the limitations are.
The code that creates this behavior isn't for the faint of heart and it's also specific to how I configured my tag panel.
$replace($regexp($regexp($regexp($regexp($sort($regexp($regexp($list(π¦,:$repeat($char(9),4),π¦,\\),π¦(?:album(?:artistsort|artists?|sort)?|artist(?:sort|s)?|barcode|catalognumber|co(?:mment|mposer|pyright)|description|dis(?:ambiguation|cnumber|csubtitle|ctotal)|genre|isrc|label|lyricist|mediatype|mb_artistcredit_artist_country|musicbrainz_(?:(?:album(?:artistid|id|releasecountry|status|type))|artistid|release(?:groupid|trackid)|trackid)|originalyear|publisher|setsubtitle|title|track(?:total)?|version|year):\t'['^π¦']'+π¦,,1),(?<=π¦)('['a-zA-Z_ #\d']'+:\t++(?='['^π¦']'{68})'['^π¦']'{67})'['^π¦']'*,$1β¦),π¦),\r?\n,β΅),(?<=π¦)('['^:']'{1','31})(:\t+'['^π¦']'{1','36})(?=π¦),\U$1\E$2\r\n),(?|(?<=π¦)('['a-zA-Z_ #\d']'{32','}:)\t+('['^π¦']'+)(?=π¦)|(?<=π¦)('['a-zA-Z_ #\d']'+:)\t+('['^π¦\n']'{36','}))(?=π¦),\U$1\E\r\n $2\r\n),(?<=π¦)(?|('['A-Z_ #\d']'{1','7}:\t{4})\t*|('['A-Z_ #\d']'{8','16}:\t{3})\t*|('['A-Z_ #\d']'{16','23}:\t{2})\t*|('['A-Z_ #\d']'{24','30}:\t)\t*|('['A-Z_ #\d']'{31}:)\t*),$1),π¦,,π¦,)
It adds a filter to this solution for displaying all tags in a single _FORMAT field early on to keep the performance acceptable (but even in the video you can see a slight flicker after selecting the third track because of the long LYRICS field while the _FORMAT field is evaluated).
However I'm sure you or 7stones can build a less complicated version if you have fewer tags in your tag panel and/or are not as picky about the horizontal padding as I am.
For me, being able to quickly flick through the tracks and seeing only the nonstandard tags in one place is good enough to find tracks that are missing a tag or have more than others.
I'll add this version with a breakdown to the linked thread. Edit: Done.