Hi Matt,
We do not yet support the WP8 API with PlayScript. It took Icer a little while to implement Stage3D support in OpenGL for Android/iOS, and we're still working on making sure the Android version actually works.
That being said, the OPENGL and TARGET_MONOTOUCH macro defines could be replaced by DIRECTX and TARGET_WP8, if you're willing to put in a little bit of work. It's unlikely we could get to this any time soon, but if you're up to it we would really appreciate the help.
Icer can work with you to work through the AGAL to HLSL conversion (required for shader support), and some of the other issues that you'll run into. At this point I suspect it is a bit more straightforward than it was for the openGL port, since Icer now has a fairly systematic way to convert AGAL for GLSL already.
In any case, let me know if you're interested.