Your chances are really good!
Make sure you're added to the list BEFORE the flight closes (usually T-60) and then go to the gate and see if you've been upgraded.
They process all the Premium Economy pax BEFORE anyone in Economy! So even a Super Elite with full paid Y-ticket will be processed after you.
I did this exact thing a couple months ago for my sister and she cleared it at gate and got Business class even though there was only 1 open seat for sale at closing.