Namkung, Ju. Phonological Inventories of Tibeto-Burman Languages. 1996. hh:hld:Namkung:Tibeto-Burman
"/a o ɤ/ are normally pronounced with
some constriction, and they function as constricted equivalents of /ɔ u ɯ/ respectively in vowel harmony rules."
"Constricted vowels appear only in the mid level and low falling tones in native words", and constriction raises the tone slightly, so it would probably be better to analyze constriction as a property of the tone.