Yes, you can prepay your MF - I did it last year when I was getting 5% rewards on all Disney purchases with my Disney premier visa. I just called MS and told them what I wanted to do. For the rest of the year, I had a $3000 credit on my dues statement whenever I looked at my account online.
When the MF were actually announced at the end of the year, I was sent a bill for the difference in my prepayment and what I actually owed.
It was a great way to rack up some extra disney reward dollars on an amount that I already knew I would have to pay.