I think it's a good idea to be wary of the idea that the GMAT will never use a common type of construction in a Sentence Correction question unless there's a clear rationale for not using that type of construction.
For example, it's logical to believe that the GMAT will never require test-takers to have knowledge of phrasal verbs to answer SC questions since phrasal verbs are rare in languages other than English and are not commonly used by non-native speakers of English.
So, is there a convincing rationale for the GMAT's never using "either" or "neither" as a subject? If so, I'm curious regarding what it is. If not, then, be careful of believing something that's based only on what the GMAT has done in the past. In that case, the GMAT could do in the future what it has not done previously.