I am not sure anymore why those mathml tags were stripped but in a quick test with moodle 2.2 I was able to add the tags to numerical and multiple choice questions by editing my profile and changing the setting "When editing text - Use standard web forms" (no editor) and selecting HTML format under each textarea where mathml was needed (not moodle autoformat).
You could try to ask this issue on Quiz forum although cross-posting is not usually the best way to get answers but in this case people on that forum might know better the reason and possible fix