So, it seems boars don't see a 1 block wide gap as pathable. With a 2 high by 1 wide opening that you can walk through, boars won't go through unless pushed by you or another boar, even with a food enticement. If the gap is a bit over 1 wide then boars will go through as easily as a 2 wide, or open door. One block high also doesn't work, but I didn't try between 1 and 2.

Piggies apparently see themselves as full size and follow the same rules, and so I think I'm stuck with having to separate boars manually. Crap.