It would be a great enhancement to make more use of the composer tag.   Unlike modern/popular music, in classical music the performing artist is very seldom the composer (in fact for composers before the 20th century the composer is *never* the performing artist!). Typically someone might have a collection of works by <a_composer> performed by numerous different artists.  And it's normal for performances by the same artist on one CD to contain works by multiple composers. 

It would be a real bonus to be able to search/filter by composer in the same way as I can search by genre/artist/album etc.  And it would be very good to allow the user to set which tag elements are displayed in the playlist window.  At the moment it displays track number, track name, artist, album.  Ideally I would like to be able to set it to also display composer.

It's mostly done in the v1.1.x branch, the only thing lacking is the gui to activate it (sorry it's taking so long :-[).

In the meantime, if you want to test it, you can use the v1.1.x branch (latest git version is better), and remove the line "_disabled=>1," following the line "composer =>" in gmusicbrowser_songs.pm.
You can then add a composer column by right-clicking on the column headers, and add a composer tab in by right clicking on other tabs. You can use the composer value in some places (where you can use %a for artist ...) by using $composer.
Of course re-reading the song tags is needed to pick-up existing composer tags in your collection.

Great stuff, thanks.  Will build a development version and try it out.

It works really well after upgrading to from git, thanks for the advice.