I searched for, but did not find the answer to this. Also could not find a role for "Orchestra" under Cast or Crew. As an Opera and Music lover, I would want to keep track of this when I catalogue my DVDs or BDs. Am I missing something that should be obvious?

I see (hear? ) the orchestra as Cast, and the credit I find most often is

eg.
Xyz Phil. Orchestra role: Orchestra
Xyz Phil. Orchestra role: Music played by
Sir Charles Mackerras role: Conductor

Should be easy to document "role" if not present, from rules:
"If an actor is credited by name but does not have an associated role, you may use the film itself, or another source to identify the role. "

Further, the rule : "For music and stage performance DVDs, use the performer or group name followed by the DVD title, separated with a colon and space."
usually is interpreted to Composer in stead of "performer or group name", as in:
Mozart: Requiem

I guess most music lovers store the DVDs, at least, divided in Music and "other", hence this way makes sense ( for me... ),
- All Mozart before all Verdi eg.

If not visually present, I use the voice only checkbox, eg. Branag's "Magic Flute" where you do not see the orchestra. Other DVDs the orchestra IS the movie, then, of cource, no checkbox.

Nice to hear there are more opera & music lovers among us 

Ok, got it. Thanks.

I would assume there is an existing feature request, no?