Here come the speculative etymology of khas ’che ba “to assert, maintain, claim” or also “to be alleged” as in the case of “alleged monk” (khas ’ches pa’i dge long)* and considered to be synonymous with khas len pa (Bod rgya, s.v. khas ’che ba). It is clear that khas is the (frozen) instrumental form of kha (“mouth” or “speech”) but what about ’che ba. Note the (archaic) orthography in the brDa dkrol (s.v. mches pa). Jäschke provides the meaning “to assure” or “to promise” but the only combination seems to be khas ’che ba but I feel that the meaning is still somewhat uncertain. It is truly speculative but I wonder if ’che ba and mche ba (“corner-tooth, canine teeth, fang, tusk”) are cognates and that ’che ba once meant “to plant (e.g. seedling) or to implant” or perhaps “to bore/dig/cut into something.” Note that in Tshangs-lha (a language spoken, for instance, in East Bhutan) “to plant” (e.g. a seedling) is also called ’che ba. And possibly mche ba (noun) meant a special “instrument of making an incision or digging/boring/cutting into (something)” (cf. “incision tooth” in English) and if mche ba had once a verbal form, then it could have meant “to make an incision” or “to dig/cut into (something).” In short, the etymology of khas ’che ba could possibly be “to plant something verbally” and khas len pa “to accept verbally.”
* Cf. Bod rgya (s.v. dge sbyong du khas ’che ba).