I'm rather sure that the suggested AC coupled 50 ohm termination can't give sufficient performance for a high impedance probe. There are good reasons why oscilloscope probes are using resistive cables since about 50 years.
There's an interesting 1969 tektronix publication "Oscilloscope Probe Circuits" available as reprint on the internet, e.g. from It has a detailed explanation, why resistive cables are achieving considerable higher bandwidths than terminated lossless coaxial cables.