I recently through-checked a bag from a UA revenue ticket to an AC reward ticket. The UA agent had to make a phone call for help though.
There would have been enough time to retrieve and re-check, but only because the AC flight ended up being delayed 2 hours. Otherwise, I'm not sure it would have been possible.
Your best bet would be to ask in TAM's forum here if people have experience doing it recently, and for any advice.