For Works, there is a "Work-to-Artist relationship" with the option of Publisher, but there does not appear to be a way to add a publishing company, is that correct?
Works also has an "External Link" option to a Score URL, but this allows only a whitelist of public domain scores. Is there a reason we don't allow a link where a score can be purchased?
Releases have a commercial label, and URLs to commercial sites for mailorder and download.
Is a publisher added as an "artist"? Or as a "label"?
You can both link artists and labels as publishers of works. The latter are even the most frequent.
OK, thanks, I see I can add a relationship linking to a Label as a publisher.
That just the published score of a work, and a URL that is not on the whitelist.
The score URL is for places where people can get/see the score freely. We have "purchase for download" URL relationships for Releases and Recordings, but I don't think anyone's suggested such a link for Works (yet). http://tickets.musicbrainz.org/browse/STYLE would be the place the suggest this if it's something you would like to see.
Issue already filed, see this other topic.