Do you consider the Eldarissa munta 'nothing' unfit for import into late-source Quenya? Etym. has the negative root variants MU- and UMU-, and -nta could plausibly derive from *nata, a by-form of nat 'thing'. 

Well, your reasoning is sound, the word fits phonologically, and it does not clash with any later word, so yes, it could be imported, I agree.