For current, a current clamp is a complete answer.
For voltage, you will need to know the dimension of the inner conductor underneath the insulation, and the dielectric constant of the insulation, but you can make a good guess for both of those.
Wind a thin metal foil over the insulation. Wind some insulation over that, and another foil, which you connect to earth. Connect a length of coax, inner to the first foil, outer to the second. Take this to your oscilloscope.
From the dimensions and plastic properties, estimate the capacitance you have cable to foil, and between foils, using the standard capacitance formula for coaxial cylinders. These, together with the capacitance of the cable, and the capacitance and resistance of the scope input, form a mostly capacitive voltage divider. You could probably estimate the gain of this to within a few 10s of percent, or you could calibrate it for higher accuracy.
If you only need a voltage waveform reference to use with the current clamp to determine power flow direction, then the absolute gain of the voltage divider is not too important. To minimise the phase shift due to the scope's input resistance, parallel a large enough capacitor with it. For instance, 100nF//1Mohm gives you 100mS, but would give you a huge voltage pot-down ratio with your few pFs pickup capacitance.
You may want to precede the scope with a FET follower, or a high impedance opamp to buffer the signal. Putting this right at your foil-wound pickoff capacitor would minimise the signal loading and improve your output signal no end.