I can't afford the figure up front right now so i've been thinking of doing this too. Half an inch is a lot at the scale though, do you think it will look odd?
Believe me, I did think about this for way longer than I should have! But a 9-inch sword with a 12-inch figure will certainly look oversized in that scale. It may not come all the way up to his chin like the real sword, but it'll be pretty darn close. I don't have the letter-opener in hand yet, so this is all speculation on my part at this point. Maybe I'll pick one up this week and check it against some other figures.