Benzyne formation with Grignard reagents
NAS reactions normally require strong electron withdrawing groups to facilitate the addition.  Normally these groups, nitro, carbonyl, cyanide, are preferentially good electrophiles for Grignard reagents (nucleophiles).

So no, organomagnesium reagents will not react via NAS.

You also mention the Benzyne mechanism (which is somewhat different).  In this case, where nucleophilic attack on other groups is not possible, I expect metal-halogen exchange due to aryl anions being slightly weaker bases than alkyl anions.  After said exchange benzyne formation is possible.