Well, tried it...and darned if unclearthur ain't right! Here are some pictures. I moved it maybe an inch forward after this:
Here is how it looked before mounting. I previously had the black blanket sticking out a few inches:
For heading straight down the road, not much difference. However, one of the things we practiced in the arena was tight turns - tight for Mia, who is a bit of a klutz -and cantering in the arena. She did noticeably better in the tight turns, and her cantering moved more freely. We've been doing some work on cantering, but we did more laps without stopping today, and she seemed to use her back more aggressively. At one point, she actually accelerated into a gallop, which my small arena isn't big enough for - but that indicates to me that she felt better while stretching out.
So overall: Excellent tip! I thank you, and more importantly, I think Mia thanks you, unclearthur! Or she would, if she didn't have her mouth full of hay right now!
Sorry for hijacking the thread a bit.