Hi Calum,
Actually it is possible to model electro-optic effects with OptiBPM. We have a tutorial on a modulator at:
https://optiwave.com/optibpm-manuals/bpm-lesson-10-electro-optic-modulator-2/
In this tutorial we reproduce the results from the paper:
S.R. Sakamoto, A. Jackson, N. Dagli, “Substrate Removed GaAs-AlGaAs Electrooptic modulators”, IEEE Photonics Technology Letters 11(10), p1244 – 1246 (1999)