I'm wondering if brush chatter could be contributing. I can't find any
log entries suggesting that they've been changed, and the thing has
about 400 hours on it.

Chris,

For what its worth, I had the same symptoms after replacing a bad
voltage regulator with a Zefftronics unit (I did this after having
chronically low voltage in the system, turned out the old VR was an
automotive one, a very bad thing with a split master switch, but
that's another story). The new regulator was keeping up nicely, but
when a large load was suddenly placed on the system (like flaps, or
landing light), the voltage would momentarily shoot up to 16V, the
built-in OVR then (correctly) tripped. Anyway, the problem was in the
tired brushes, I had to get it going ASAP so I just swapped in a
rebuilt alternator, but I am sure brush replacement would have fixed
it also.

Martin