Hi,
Well, Forest can't access these VRayProxy object settings (like the
Proxy mesh visibility), but I would suggest you a work-around for such purposes by using the
Forest Color map.
Each "object" inside of the VRayProxy object should use a different material and by wiring the
Forest Color map to the
Opacity slot of such material and using black & white maps you can drive its visibility (& tweak its probability). Attaching screenshot of a possible set-up.
Hope you'll find it useful.
Best regards,