Yes, they are one and the same (the Beirut plane and the 'last flight' plane). I took pictures to prove it, and it was confirmed by former pilots who were on the last flight (which was MSY to STL).
I think it was planned. N54339 was still in the fleet at the end, and it seemed the obvious choice to me.
Tom in NO (at MSY)
"The criminal ineptitude makes you furious"-Bruce Springsteen, after seeing firsthand the damage from Hurricane Katrina