℗ rights relationships are presently not available to recordings, but should be made so: [label (typically a record company)] => ℗ rights => [recording]
℗ rights should be attached to recordings, no matter where these recordings might be found. the addition of multiple ℗ rights over time may be possible—eg, when vinyl releases are reissued in cd format, we often see revised ℗ rights dates. but that shouldn't change the nature of the recording; it is still very much the same one.
the present state, where these relationships can only be added to releases, is problematic, because it isn't really the packaged releases that are being protected by these rights, but rather the sound recordings they contain. for a studio album released by an artist, the problem isn't evident. adding the ℗ rights (with common date and rights holder) to the release just implies it is true for all recordings contained in that release.
however, for compilations where, often, different ℗ rights (in terms of both year and rights holders) apply to respective track titles, this becomes a very obvious problem. the multiple ℗ rights could be added to the release, but then there is no way to identify them with the specific tracks/recordings they are intended for. in the end , we are left with a list of ℗ rights with different dates and companies that has little meaning.
additionally, it is a highly inefficient way to incorporate this data into the database; just as with all other recording-level relationships, any updates/additions can be localized there and, once edited, be immediately available to any releases containing those recordings, cf. the present way of adding this data to multiple versions of an album, ie multiplying the number of instances where edits would be needed to enter a data common to those same recordings.
related (once was part of): http://tickets.musicbrainz.org/browse/STYLE-315