I never really took the time to answer my own question here, so I think I'll pluck this little gem from page 3 and put 'er back on top:
I think Luke would have lost. And I see two or three scenarios in how he would have lost.
1. Luke would have been presented to the the Emperor. The Emperor would have tried to turn him- first by talking to him and then attacking him. (All very much in line with the events of the end of RotJ- except for the absense of Vader) Luke would have stayed cool, however, and not taken the bait from the Emperor. The Emperor would then realize that Luke was not going to be turned, so he must be destroyed (again, like the ending of RotJ) and Luke would die. Perhaps he gets to be a Force Ghosty and hang out with Ben and guide Leia into becoming a Jedi.
2. Luke would have been presented to the Emperor. Luke would be ready for the clean kill- a proper "light side" way to rid the Galaxy of the Evil Emperor. But before he gets the chance, the Emperor begins to goad him and make him feel bad/fell mad. Then he justifies his strikes against the Emperor and his usage of maybe just a little bit of the Dark Side to strike him down. Luke has the Emperor on the ground and poises to strike him. He is full of dark side energy.
2.a A look of peace crosses Luke's face. Reluctantly, Luke lowers his arm and shuts down the sabre. He bends on one knee in front of the fallen Emperor. The Emperor rises to his feet and demands fealty from young Skywalker. Luke, corrupted by the Dark Side, swears allegiance to his new master. He watches as the Emperor returns his attention to the battle out the window. Luke joins the Emperor as they use the dark side to guide the Empire to victory.
2.b Luke has the Emperor on the ground and poises to strike him. He tries to push the dark side from his mind. But his emotions outweigh his thoughts- he angrily swings the sabre and slices the Emperor clean through. Luke gets ahold of himself. He has done it. And it did not, as the Emperor warned, cost him his soul. He has not only rid the galaxy of the Evil Emperor, but he has used the Dark Side of the Force and bound it to his Light Side will. He has nothing to fear of the Dark Side. It is his ally. It is under his control. And he can bend it to his will. It will not control him. Ben was a fool to think it would. Yoda wasn't strong enough to control it. Not like Luke is. Luke is truly the master of the Whole Force. And with it, not only will he bring down the Evil Empire- he will bring peace to the Galaxy. He will bend the galaxy to his will, just as he has done the Dark Side of the Force.
2.a - I think this is really hard to pull off. When the Emperor says things like "Strike me/Vader down and your journey to the dark side will be complete!" I think Luke must be thinking: "I bet I can strike him down and not be totally won over to the Dark Side... Why would killing one or two evil men, even in anger, makeover my moral view of the galaxy and instantly make me join an enemy? He can't be serious!" I can't really see Luke deciding to serve the Emperor in the near future no matter what happens in the Emperor's throne/thrown room.
2.b Rather, I think Luke would kill the Emperor, with the Dark Side, but not chose to serve him. He would emerge from this experience less afraid of the Dark Side. More sure of himself. Palpatine and Yoda both said that just a little taste of the Dark Side would change Luke forever... Well- they were wrong! It didn't change Luke. He was the same as he ever was. In fact, he was better! He no longer had to fear treading down the dark path and becoming his father. He was above that! And of course, it would be these feelings of triumth and disregard for the warnings he had been given that would ultimately be his downfall. The Emperor taunting him about instantly becoming a servant of the Dark Side would cause him to focus on the near term effects (which don't really exist) and ignore the long term effects.
The 2.a scenario is along the lines of Anakin's fall in RotS. I can't for the life of me think of how this single scene could be changed to make it end with Anakin joining the Dark Side full time. I can see him momentarily employing the Dark Side to get himself out of a jam- and then maybe losing his fear of the Dark Side to where he's using it a lot... and then finding that he doesn't agree with every point of Jedi doctrine... being outcast from the Jedi once they hind his usage of the Dark Side to be unrepentant... and before long... he's a Dark Jedi.