As others have already said, it's very unlikely. They only op-up if they have to, which means it happens only a) when economy is oversold, b) everyone that wants to upgrade with miles has been upgraded, and c) there are empty seats in BF. Even then, it's typically done by status, so if you're anything less than a Platinum, there's almost no chance of it happening to you even when it happens on your flight.

CO is generous with upgrade inventory: they'll offer every available seat in BF for upgrades and clear them up to the moment that the door closes, so it's very unusual for an empty BF seat to go out with someone in the back waitlisted for an upgrade. This is different than many other carriers, which limit upgrades and will let premium seats go out even though there are people wanting to upgrade. But, once all the paid upgrades are processed, they are very strict about not giving them away unless they have to for operational reasons.

I know of a situation on the former NW where a passenger was upgraded on a long-haul int'l flight because his flight attendant friend put in a good word with his buddy the gate agent. On CO, it's been reported that each and every upgrade to BF is carefully scrutinized, and that it's a termination offense if an employee gets caught upgrading a passenger to BF without justification. So sweet talking is very unlikely to get you anywhere.