Unfortunately we will not be able to implement this feature anytime soon, so you should look into the platform code yourself if you need it now. We have a pending dot release of CORE that has loads of new features that needs to be finished, as well as other products that take precedence at the moment.
Obviously I haven't written the code yet, but it's very likely you'll want to add a hook on Windows that monitors for WH_MOUSE_LL events, and you'll want to create an IOHIDManager on Mac with IOKit to receive events for the kHIDPage_GenericDesktop/kHIDUsage_GD_Mouse page/usage pair. If you don't use low-level events like these then it's probable that Illustrator will consume the event and you'll never receive it at the plug-in level (this is what happens with other events like keystrokes).
Obviously I haven't written the code yet, but it's very likely you'll want to add a hook on Windows that monitors for WH_MOUSE_LL events, and you'll want to create an IOHIDManager on Mac with IOKit to receive events for the kHIDPage_GenericDesktop/kHIDUsage_GD_Mouse page/usage pair. If you don't use low-level events like these then it's probable that Illustrator will consume the event and you'll never receive it at the plug-in level (this is what happens with other events like keystrokes).