Are you talking about buying an upgrade (you can do that at check-in if there's space), or bribing the gate agent?

If the former, yes, you can do it.

If the latter, I don't want to say it will never work, but on a flight that's long enough that you'd be willing to pay "a couple hundred", but not do a real LMU, there are going to be multiple gate agents (watching each other). And I don't think any of them would want to risk getting fired for a couple hundred dollars.

Just imagine if you were on the upgrade waitlist, saw this happen, and didn't get your upgrade. I would make a huge scene at the gate, and I would do everything in my power to get that GA fired.

So in short, don't count on bribes to work.