Well, the acceleration does work, but not very well.
It appears that many + or - events are discarded, more so when the CPU has a hard time keeping up with everything else. At least that’s the impression I get on my system. I have found that if I turn the knob at a moderate speed for more than a second or so, the acceleration kicks in.
I agree that switching between the two modes manually would be a far better solution. As you say, there is no binding for that in the GUI, but from what I’ve heard, it’s available through FSUIPC7 and other add-ons capable of accessing the inner workings of the sim.