Lil update:
Unsurprisingly, the new El Cap beta didn't make my tablet suddenly start working.
Then I got scared that maybe the tablet was broken, so I reverted my Air back to Yosemite and it's working again. So it does seem to be an issue with El Cap + my machines + the drivers.
Can you share the instructions you used to revert back to Yosemite? I'm having the exact same problem with a Cintiq 21UX and no amount of deleting and re-installing the drivers seems to help.