Tag, hun, I must correct you.
A swing, TECHNICALLY is an ensemble member. The contract is a Pink production Contract (ie, ensemble contract). So, by contract definatition, there are ensemble. They just do not perorm unless a member of the performing ensemble that they cover is out.
Not all 'tracks' require an annnouncemebt or any sort. Ensemble roles do not. Some shows, such as Mamma Mia, elect to list them on a board in the lobby with principal understudies, but they do not HAVE to. Only principals and 'specialities' such as the Dynamites in Hairspray or the Moth in Tarzan are required to have announcements made for replacements. Announcements do not have to be made for ensemble.
There must be two types of notification used, but which to is up to discretion of the production. Lobby board, Playbill insert, or PA announcement. PA announcement is least common because it is the most likely to result in audible groans... and also easiest for audiences to forget.
When Hairspray opened, the production had to dsicuss announcements with equity because all tracks in the show had character names, but equity ruled that they only have to annnounce principal understudies and Dyanmite understudies (though they never specify which one is one.. you have to figure that out on your own). The first National tour, however, chose to list all ensemble replacements on the lobby board alongside the principal understudies.