Cable for remote capacitance measurement
Since I'm not interested in absolute value but only dynamic change in measurement, any coaxial cable with fixed capacitance-per-length should do, right?


Well, to a point. If the dynamic change becomes too small a fraction of the absolute capacitance, your signal will get lost in the noise. You really do care about all of the capacitance connected to the device pin, which can only measure the total capacitance that it sees.



There are ways to measure capacitance (or in general, impedance) at the end of a transmission line, but they require more sophisticated circuitry such as TDR or the equivalent frequency-domain algorithms.