After trying this for a while I decided to take a look at the Ninja project. Turns out, that guy is cheating - he’s using a Generic rig, not a Humanoid one! Switching over to a humanoid rig breaks the whole Animation Rigging system, in that example project and in my project. What’s going on? Isn’t this supposed to be working on Humanoids?
Quick forum search reveals:
a) people having the same issue with no reply for a month now
b) an answer from Unity that it “should work” on Humanoids