The point of this is to avoid scanning content again, so first we'd have to save sound and subs like we do video. Initial motivations are

a) to allow just a metadata re-write; quite easy, just don't do any Player or anything
b) to allow addition of subs from .srt to an DCP that you've made already; then the player would have to happen but without playing certain content

Suggests that you need to tell player not to play certain stuff, like is already done when referencing existing DCPs. In fact it's quite similar, Writer wouldn't receive any data then it would get the MXFs by copying from the store (instead of linking from the OV).