I highly suggest you attend some rape prevention courses. If a woman is being attacked (and Rape is about power) she needs to do everything she can to survive. Her attackers well being is the least of her worries.
A woman on her back and outweighed by a hundred pounds did not get there with her consent. She is more than likely already suffered some serious physical trauma and under some form of extreme emotional and/or physiological duress (and is perhaps already in shock.)There are specific simple guidelines to follow under such circumstances and perhaps the least of these is some type of choke or "wrist lock from the guard"...
Very few Dojos teach Rape Prevention the way it should be taught. Indeed 90% of the Rape Prevention Classes I see in Dojos are highly dubious. Your best bet is to invite a Local Law Enforcement Agency to come teach a class. Then perhaps you can exchange ideas about how to incorporate your "style" of practice into their training.
There is much much more to rape prevention than Waza my friend.
Back to the subject.
I have attended some rape prevention seminars. Date rape is the most common form of rape and these women find themselves on their back. So yes, a good arm lock or wrist lock for an unruly boyfriend to get off of you may work.
I'm not an expert and that's not the point here. Only to suggest that it's an option for self defense.