Short answer: you can't trigger greenfire blade at a distance.
Long answer:
Nice idea (I got enjoyable flashbacks, thanks ^^), but as I see it, when you make your melee weapon thrown, it stops being melee. You could do several things to increase its reach, but it does have to be in your hands. There ARE powers that make you throw the sword, but that's fluff to explain it being a weapon and not implement power.
You could make your own homebrewed power, ranged 5, weapon keyword, Int vs AC. You teleport (or just jump) on top of the creature, stab it with your weapon, teleport back and cast a lightning bolt on the sword. It disappears and reappears on your hand.