Thanks a million for all the great ideas!
I especially liked the trilobular screw threads idea. I looked them up
on the Taptite site:
and they are remarkable little critters indeed! The binding head
screws are also a good fit for this project; I'm going to see if anyone
makes a trilobular thread on a binding head screw. Several of you
mentioned star washers, and that is also a good solution although it's
either/or with the binding head screws.
Loctite is your friend, but in this case I need electrical continuity
and multiple releases & reinserts, so I'd better pass on a liquid
thread locker. Also, I've got no access to the rear of the busses, so
peening the screw tips once inserted wouldn't work. I really liked the
safety wire idea and the tablock, but the customer is low-tech and I'm
unconvinced they would reliably restore the safety wire or tablock
after maintenance work.
Thank you all for the help!