We fixed it!
It turns out, a lot of these new alloy wheels don't sit flush on the axle, and need specific o-rings to fill the gap. The wheel dealer had left these off, so the wheels didn't have a central pivot point -- hence the high-speed wobble.
After 6 full wheel balances this is a huge relief... and although the wheel folks were more than helpful, I still can't help but feel a bit angry that they didn't pick it up sooner.