Just use any garden variety epoxy putty. You know, the stuff that looks like a toxic twinkie with 2 parts in a roll form. Knead some up, wrap it around the underside of the head and stuff it into a countersunk hole. Then stuff another gob down on top of the bolt head and use a putty knife to scrape it flush before it hardens. You can even sand it afterward if you need to. Those bolts shouldn't budge after that.
RandyT
Thanks for the advice. The stuff I used was the stuff that comes in the hypodermic syringe looking thing. That's what I assumed you were talking about, but now I notice that you said in "roll" form, rather than in "tube" form. At any rate, it's rated for 2500 lbs. which was the strongest stuff I saw at Ace, and the packaging said it was good on metal and wood. So it seems like it should work.
I'll be heading back to buy some more today, though. The stuff says that you can work with it for half an hour before it sets, so I mixed up about 3/4 of the container for the work I was doing.
Five minutes later it had hardened to the point that it couldn't be used.

What I was able to use is all set up now, though, and it looks really strong. BTW, I also looked for Bondo while I was there -- not for this job, but just to have on hand for repairs and gap filling, etc. Couldn't find it. Should it be in the same section as the caulks and adhesives? Does it come in a can or a tube?