About Zhuang: according to Luo Yongxian in the Routledge Tai-Kadai volume, the most general negative word is bau (mid-tone), and this is “found across the Tai language family.” If that’s the case, maybe it’s not worth speculating about เปล่า as a content word that grammaticalized. Instead, we might focus on Thai ไม่ as an innovation or borrowing. I checked the entry for 沒 in Axel Schuessler’s ‘ABC Etymological Dictionary of Old Chinese,’ but no leads there. By the way, Schuessler pays quite a bit of attention to mainland Southeast Asian languages in his etymologies, esp. Austroasiatic.