I finally had a chance to go look at my bee-infested truck again today. It's been almost a week since the poisoning attempt. This time I felt bolder in my home-made bee suit (military surplus mechanic's coveralls, mil surplus insect head net, and some gloves), and I got in a lot closer to the area of interest.
I first noticed much less activity than before. There were no bees loitering around the opening, and just a few flying around. I don't know if that's because the poison made a difference, or if everybody was just out working. I saw a bunch of dead bees up on top of the truck bed that I hadn't seen before.
This time I got close enough to get a much better look at the hive opening. I was expecting it to be a gap in the wood strip, but it really is a gap in the bed around the mounting bolt like I first suspected. From the looks of it, if I need to try poisoning through there again I should be able to pinch off then end of the copper tube I used enough to cram in there. Maybe I managed to get enough poison up into the hive to mess things up well enough already, though.
This time I tried sealing the hive opening with spray polyurethane foam. I'll let it sit overnight, and see what's going on tomorrow. Maybe they'll find another opening, or maybe the hive is already done for from the poisoning.
New pictures attached.