I was having a look in that file, saw that there was a workaround for the Latitude E7440, and thought whatever it needed was bound to be necessary on my E6440 as well. And maybe it was that partition error message that I get at boot.
This led me to bug #194359 , which sure sounds like it. So I gave the offered "gpart set -a active [disk]" a try, and it worked! No more "Invalid partition table!" at boot. Which also leads to a documentation question of: If there's anything in gpart(8) about setting active on an entire drive and not just an MBR partition, I don't see it. So is it meant to be somewhat secret/undocumented, as it's only for certain screwy systems?