Can I use a XTR 9000 power meter on a XTR 9100 crankset?

Great question! Unfortunately the answer is no. Shimano changed the spline depth on the x100 series. Therefore you would need a XTR 9100 specific power meter such as the Stages Shimano XTR M9100/M9120 Power Meter or 4iiii PRECISION Shimano XTR M9100 Power Meter. You could also install a spider based power meter such as the power2max Shimano MTB Power Meter.