I"m not sure that any of us should be condoning this wicked act of Wayne"s, by providing him with ideas of how to get out of a meal with his Mother-in-law.
However, if you"re insistant, it"s going to have to be something that will allow you to get away with Mrs NoseyP coming home and finding you online and playing poker.....
Not sure that a headache will allow that, I"d suggest that you concentrate on the stomach ache and a touch of diarrhea.