I would start with Section IX, Article QW-304, from there go to QW-191, note the exceptions taken for certain P-numbered materials. Then one must go to Article 191.2 for UT. That bounces the reader to Article QW-142 that takes the reader to QW-304. Welcome to ASME folks. Now, go back to QW-191.2.1, as noted, UT is only permitted for materials 1/2 inch thick or thicker.
So, in response to your inquiry, in my opinion the answer is UT cannot be used to qualify welders if the base metal is not at least 1/2 inch thick. There is no exception that I saw in my review of Section IX.
Best regards - Al