I have just stumbled upon this site, a list of all the hidden songs.
It was created 20 years ago, apparently https://hiddensongs.com/about

Should it be linked to songs?
Should it be linked to release groups?

Should it be linked to releases? As all editions have not the songs hidden, usually.
Most of the time yes, but sometimes, later releases omit the hidden songs or make them plain in sight visible.

Example https://hiddensongs.com/david-bowie-david-bowie could be linked to the original releases but maybe not to https://musicbrainz.org/release/17ca3011-430c-3063-af7b-4465fc82ec91 (tracklist does not show it but historical booklet mentions it) and certainly not to https://musicbrainz.org/release/841f5313-1f80-49ac-af21-4e992cc82b17 (where it's printed).

But maybe it would be less stressful to simply link it to release groups?