Thanks for the answers! I'm now with the 3.1.6217 version.
I'm using these tags in the browser. I have 3 columns: Composer (which is actually <artist>), Year and Performer (actually <album artist>). The multiple values are OK. An album by Sonny Boy Williamson & Memphis Slim is tagged as such:
Artist: Sonny Boy Williamson & Memphis Slim (compatibility tag)
Composer: Sonny Boy Williamson
Composer: Memphis Slim
Album Artist: Sonny Boy Williamson & Memphis Slim (again, for compatibility)
Performer:Sonny Boy Williamson
Performer: Memphis Slim
In the browser, the <composer> and <performer> columns display one line each for "Sonny Boy Williamson" and for "Memphis Slim" and none for "Sonny Boy Williamson & Memphis Slim" so that's perfect. But they're not sorted, even with the latest patch.
But with this new patch, creating a virtual tag for <composer> works ( $Sort(<Composer>) ). The column is both multivalued and sorted. I still have the problem with the Performer column, though:
Using the <performer> tag as is, the column is multivalued but not sorted.
Using the virtual tag $Sort(<Composer>), the column is sorted but multivalued tags are not recognized, so I have "Sonny Boy Williamson; Memphis Slim" on one line, separated by the semi-colon.
I guess that's because <composer> has been made sortable by default, just like <artist> and <album artist> but <performer> being a custom tag, hasn't.