To answer my own question --- I still don’t know!
I left in Brazil the suitcase I was going to check (until my Nov. trip), so the problem did not come up. However, once I got airside, I found several gate agents, who were momentarily free, and talked to them about this. They indicated that the check-in agents would check-forward luggage only if you have a boarding pass for the ongoing flight! But the problem is, for an international flight, unless you are in the same PNR, you cannot get that BP in advance! I’m still not sure if this is correct info.
That's simply not true (surprise, surprise; we've been given inaccurate info in airports many times). When we've flown FOR to GRU or GIG on TAM, and then on to the U.S. on another airline (UA, US or AA), we were on the same PNR but when we checked in for the domestic flight, we could not get the boarding pass for that second flight. Our bags were always transferred, but, as I said, we had to leave the secure area to stand in line and get that boarding pass. TAM checked our bags straight through but perhaps would not do so if both flights weren't on the PNR.