Along the same lines as the PP's advice, possible. Not likely, but possible, depending on how busy WDW is during your trip.
You will have to be prepared to wait. Perhaps an hour or more, again depending on how busy they are.
And the advice about arriving before they open or in the last hour of service is sound, too. NO way it is going to happen between 6 and 9.
They open for dinner at 5, so if you're going for early, I'd say get there no later than 4:30 and start asking. Your best shot at this time is to get seated when they open at 5, if they have a no-show on someone with an early ADR or are feeling like doing you a favor and slipping you in.
The service officially ends at 10, so if you're going for later, you might be able to get seated after about 9:15. Keep in mind, though, that lots of people book in the 8-9 range to be able to be there during Wishes (if that goes off at 9) and so they're going to keep their tables until that is over - but then there will be a little wave of people leaving, once the fireworks are over, so you have a shot. They will not throw you out at 10:01 if they do seat you; if you're in, you'll get the same dinner and service as everyone else before you did.