I actually thought about doing this at JFK. Buying a cheap o/w JFK-BTV on the RJ (Gate 23/25) and then boarding a CLE flight at the same time
The way it works on most Delta CRJs is your BP gets scanned, then you're released onto a very long ramp where there's a ramper who you tell where you're going and they point you to the right position. Sometimes F/As check boarding passes on boarding, and they announce cities multiple times, but if the plane had empty seats (seatcounter), I bet I could do it.

-A