Digital potentiometer analogue for high voltages
If you can get away with a limited number of values and, providing there is always a net positive voltage on one wire with respect to the other, you could use a MOSFET for switching an extra resistor in circuit. This could be done via an opto-coupler but it has to be a low power mosfet else the open circuit (MOSFET off) drain-source capacitance could be a problem with the data edges being corrupted. 

I'm not sure what resolution you can get away with (and still obtain the desired functionality and performance) but I think you can obtain octal packages and you could arrange the resistors so that the biggest value is 100R, next value 200R then 400 etc.. Basically you would create a DAC with dial up resistor values. Maybe 6 bits would be enough?

It's probably feasible to use Darlington transistors too (providing of course there is the DC bias on the circuit lines you mention. They definitely come in octal packages and would have significantly smaller capacitance than a MOSFET.