Yes, you are right that the ring modulator does not need to be fully simulated in CHARGE (We no longer use DEVICE as product name). This is because, the modulation section is only a portion of the ring modulator. You can simply simulate a cross section and get the index variation under different voltage. You know the arc length so it is easy to create a phase modulation from 0 to pi.
Is the ring fully doped? if it has a portion, doing the above simulation can give you accurate result. If the doping is for the whole ring and the voltage is also applied to the whole ring (Less likely), you can build the whole ring for CHARGE.
Current INTERCONNECT may not support the whole ring modulator as it needs coupling and modulation, usually with 2 arms in opposite voltage to be efficient.