Seems easy, but there are complications. Yes, I can add new action to combo(deadly strike) for enabling/disabling mapper. And yes, mapper already supports combo(deadly string), but mapper must be enabled for combo support. You need mechanism before mapper.
Two solutions come on my mind :
1. This can be done if you combine touchscreen with your keyboard – you place combo widget on the screen and configure it to newly developed action. This action can enable/disable mapper.
2. I can add special option to Mapper or to advanced settings, I am not sure which place is better. Here you can define reserved keyboard key which can enable/disable mapper. This reserved key must be evaluated before mapper (separately) and will disable/enable mapper.