I just tested this with 5D III, 100L, and Kenko 1.4x "blue dot," and nope, it still doesn't work. After the "workaround," when I press shutter, the camera STILL freezes up, with taking the battery out the only way to revive it.
Since this topic is still coming up, I thought I would repeat my experience, which I described in another thread.
First, I would emphasize that the AFMA I'd done had nothing to do with the lens on which I wanted to use the TC. Since there are only two lenses involved in my case, my experience is fairly simple to explain. I want to use the PRO 300 DGX 1.4x only on my 300mm f/4, which I was able to do when I first got the 5D III. Later, I bought the 40mm pancake. Eventually, I fooled around with microadjusting the 40mm. It is the only lens for which I have ever made any AFMA adjustments. Some time later, I put the 300mm+TC on the camera and experienced the lockup requiring removal of the battery. I realized after a while that the only major change to the camera settings since the earlier success was the AFMA for the 40mm, and after some experimenting I was able to get the lens+TC combo working again.
What I found I had to do: With the 40mm (the only microadjusted lens, in my case) on the camera, I turn AFMA to "Disable." (As far as I know, you do not lose registered AFMA values by doing so.) Then I take a shot with the 40mm and the new setting (disabled AFMA) before turning off the camera and switching to the 300mm+TC.
I included the step of shooting a frame with the new setting. To be honest, in the middle of writing this I put some wear and tear on the lens mount by doing a series of tests to doublecheck the procedure, and I was able to get it to work without that extra step. But there were times in the past when it seemed necessary, so it wouldn't hurt to include it. Now that I think about it, maybe there were times when I tried to go between a third lens and the 300mm+TC, and for some reason it was necessary to put the 40mm on and take a shot in order to make the disabling "count."
As I've said before, I have no idea what happens if there are AFMA values registered for multiple lenses. I don't know if it would work to disable AFMA with any one of the lenses on the camera, or maybe to take a shot with each of the microadjusted lenses.
In any case, for anyone looking to get this combination to work, I hope this helps.