chevvron, only just now seen your "Edit: delete Brasilia", whilst intending to remove my subscription to this thread.
Although I tend to agree with your sentiment "who wants to go there anyway?" (certainly a day was plenty enough to view its architecture), you were correct, Brasilia - and along with Belo Horizonte - was also served by shuttle flights from Rio's SDU airport.
As would be expected, the frequency to BSB & BHZ wasn't comparable with that between SDU & CGH, being perhaps just every couple of hours, but nonetheless, the "guaranteed seat / back-up flight" was still applied.
Tickets were immediately obtainable at all Ponte Aerea airport desks upon production of the fare and ID Card/passport.
Upon once asking why onboard ticket purchase wasn't available on the actual shuttle flights, I was told, "we are afraid that too many passengers might 'forget' their wallets!"