Compensate centrifugal force in accelerometer measurements
This will not answer your original question, but it might solve your problem.

I have two suggestions, assuming you're not interested in the shaft's angle at all.

A: I guess that, even tough the centrifugal force is higher than weight force, as long as the sensor is mounted somewhat near the center, the ratio between the two would not be very high. If this is the case, you'll have, for a constant speed, a constant force (centrifugal) and a variable force (weight). If their ratio is not very high, you might be able to associate the sensor readings over time as a sinusoidal signal. If you measure this sinusoidal frequency, I think you'll be measuring the shaft's speed.

Be aware that, as speed increases, the ratio between the forces and the sinusoidal signal's amplitude and offset will vary, to an extent where you might not be able to measure the peaks due to low amplitude and high offset (basically a flat line).

B: If you mount the sensor far enough of the center, you'll clear the weight force out of the signal. If you manage to implement a low-pass filter, you can compare the mean force with a calibration curve.

Again, as the speed goes down, the weight force might become significative. This approach requires calibration every time the sensor is moved around the radius.

You might try to combine both approaches with a smart algorithm switching between the two.