It looks like the tag frame 'Ensemble' has been gaining ground for tagging classical music.
Of course it is easily added by using a custom tag, but I think it is a suitable candidate for being available by default.
id3: TXXX/ensemble
Vorbis: ENSEMBLE
And then perhaps making it a 'multiple, split-able' tag?
So then it could be used both for orchestras, string ensembles, choirs, horn sections, possibly performing together on a song/composition.
Ideas/comments?
I am not quite sure about this myself b.t.w.
Perhaps these ensembles should go under the existing 'performer' multisplitter tag:
Performer: Memphis Horns (horn section)
Performer: Kronos Quartet (string quartet)
Performer: Royal Concertgebouw Orchestra (orchestra)