Mahler's 1st symphony : Offstage trumpets in the
first movement
Mahler's 2nd symphony : Offstage brass ensemble (horns,
trumpets) and timpani in the last movement
Alpen sinfonie (R. Strauss) : Horn ensemble in the third
part (The Ascent /Der Anstieg)
You also have this kind of effects in Opera's such as
Madama Butterfly (G. Puccini) : in the second Act "Coro a
bocca chiusa" there is a viola d'amore playing with a choir
offstage.
I hope I helped, i'll try to find other examples.
Good luck and keep us updated !
Victor ROSI