@User: There are no independent device models for isolated p-wells; it should be sufficient to drop a deep nwell under any standard cell layout and surround it with an nwell ring and call it done. Magic is supposed to handle the extraction of that correctly (but it is not a configuration that I have tested much).

Would the diodes between the p-well and n-well be modelled then? Obviously both the DC path through the diode and also the capacitance of the p-well to n-well could affect behaviour of the circuit.