Here's the final stupid thing I decided to try in order to get the extra action buttons to work, which is seemingly working (based on one report):
https://github.com/tullamods/Dominos...aActionBar.lua
Basically, I created a secure frame, attached it to the ExtraActionBarFrame so that I could detect when it was shown/hidden by Blizzard code. Then, I attached the ExtraActionButton to my own secure frame, which is shown/hidden by the detector frame.