Is there any reason why a capsense button cannot be placed in the centre of a radial slider? Assuming enough ground clearance is left around the button.
The CapSense slider does an interpolation of two (or more?) adjacent sensor pads. A button in between could destroy the linearity of finger movement -> CapSense value. A way out could be to use two smaller sliders to the left and right of the button and offsetting the slider values yourself accordingly to get a precise answer for the finger position. But I have to admit I did not give that a try yet.