This is already a feature. Change your joystick to "directional pad" > Advanced settings (cog icon) > Directional Pad Layout > Analog Emulation. Now you can adjust the pulse sliders until the output feels right.

Originally posted by Moskeeto:

This is already a feature. Change your joystick to "directional pad" > Advanced settings (cog icon) > Directional Pad Layout > Analog Emulation. Now you can adjust the pulse sliders until the output feels right.

I've tried this before, but it's not clear to me what the sliders actually do. In my experience, it always behaves like a very fast turbo, or might sometimes slow down at full deflection. I wish the configuration screen explained the settings more clearly. Do you understand how it works?

Gave analog emulation another whirl just now. As far as I can tell, the turbo rate is set directly by the "Pulse time" slider, and not affected at all by the analog deflection. This makes "Analog Emulation" not a solution to my request, which is specifically to be able to control turbo rate with an analog input.
I appreciate the suggestion, though!