Use 4 optoisolated current sources/sinks under microprocessor control, and sense the terminal voltage/current at each of the two ports. Apply the appropriate passive device defining equations in software to simulate a floating inductor, capacitor, negative impedance, etc.
Something like this maybe:
PWM output goes to the NPN current sources, op amps scale and bias the sense voltages to be in the microprocessor's ADC range.
Your circuit is not fully isolated, which may affect the results you are after. If you were able to isolate the op-amps as well (isolated amplifiers are quite expensive I know) then it would not interfere with the circuit into which you were placing it. Otherwise quite a neat idea for the lower frequency range (definitely not RF).
A Howland type current source can implement the voltage-to-current function mainly effectively; you only need a couple of those (one dual op amp) to make your voltage-output DAC do two bipolar current sources. If op amp inputs can sense the voltages, the compliance range of the current sources isn't so large an op amp cannot handle it.
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