Yeah, it does seem like a lot of work. It's definitely a good idea though.
One thing I would suggest though; this is RM, and it's not easy to get people to play a game once, let alone more than once. If you want to make replay value a feature of this game, then I think you would have to do even more than what you are suggesting. Basically, it's a great idea, but you need to do a lot of other stuff as well.
Baldur's Gate is a fantastic example of what you are suggesting. It has exactly the same thing, where characters will say different things during cutscenes depending on who they were. But it also goes a lot further than just different dialogues for different characters to give it replay value, with lots and lots of optional quests and stuff.