After some continued poking around with the violin ensemble, I've found a timbral shaping setting that I think I like:
This cuts the strident middle harmonics by default and divides the timbral automation into three groups. Group 1 (#1, #2, and #3) is CC91 by default. Group 2 (#4, #5, #6, and #7) is CC92 by default. Group 3 (#8, #9, and #10) is CC93 by default. (Check the manual to see how the engine assigns these groupings -- it's intuitive, but not immediately obvious.)
I've reassigned the third group to be on the same controller (CC92) as the second group. So I have CC91 controlling the gain on the low harmonics (#1-#3) and CC92 controlling the gain on the higher harmonics (#4-#10).
This lets me adjust the timbre along two axes as I play. (I'm using a Leap Motion controller for this, so I'm mapping CC91 to left/right and CC92 to up/down hand movements.) Increased low harmonics and decreased high harmonics gives me a dark, muffled sound. Increased high and increased low gives me an in-your-face sound that's loud and has a lot of bite. Decreased low and increased high gives me a thin, clear sound. Decreased low and decreased high gives me a weak, almost sordino kind of sound. Right in the middle gives me the default that I set up on the timbral shaping page.
I'll probably clamp the range of values coming from the Leap Motion, because the extremes are pretty extreme, but I'm quite happy with how this is coming together.