This is a great idea.
Give everyone the option to put a higher weight to thier upvotes for users with lower SP ( multiplier ) as you say. This would decrease your voting power at the multiplier used , thus making it cost more to do these kinds of votes. This would give users more pull when it came to giving rewards , I think most would use this feature including me. even for me at a base upvote worth around 11 cents (more then most) it seems very appealing. I may do a follow up post on this idea and really get into the details.