Often when I've used the sostenuto pedal (in fact, whenever I've used it thus far) it's as a "helping hand" to sustain various notes individually that would be much more difficult to hold otherwise.
However, just for curiosity's sake I've yet to find any pieces where the composer makes explicit use of this pedal by marking it on the score - can anyone cite any pieces where this is the case?
There's one sostenuto pedal marking in Bartók's 3rd piano concerto, measure 75. It's in a slightly small type, though, so it might be added by an editor. I don't recall any markings from earlier composers but you'll find them in modern literature. There are many examples in Ligeti's etudes, like in L'escalier du diable, measure 26.
As a side note, Bartók's 3rd concerto also includes a sub-contra G (i.e. the G below the lowest A of a standard piano) in measure 730.
Samual Barber, Piano sonata, op.26, mvt.IV. There are several long notes in the left hand, and in one point explicitly written "sostenuto Ped.
Some Barber songs also use the sostenuto pedal: "Le clocher chante" from the "Mélodies passagères" and "The Desire for Hermitage" from the "Hermit Songs".
Grainger's "Blithe Bells".
