All righty, I figured this out! It's important to also disable the autofocus function on the AF-ON button in order to let that button enable autofocusing. C.Fn IV: Operation/Others, set to disable. With the other change to C.Fn IV: Operation/Other...
Good answers here, but I'm having an odd problem - I spent the afternoon trying to get my back button (*) set up for back-button focus, without success, until after a lot of fooling around I got the camera to show the focus lights, but it only worked...
