The meta question was prompted by the recent submission of two questions, one about the Oxford English Dictionary (aka OED) and one about the existence of a dictionary of African-American Vernacular English (AAVE). Neither of these questions exhibits any connection wit literature, neither based on the conventional definition of literature, nor on our site's lax definition of scope. (Of course, arguing against these questions based on the site scope's implicit definition of literature is unsatisfactory in a discussion about site scope, since that line of argument would easily become circular.) Neither of these questions has any relevance to literary analysis or literary appreciation.
I think we need more accurate terms to describe those questions: the OED question is about lexicography; the AAVE question is relevant to both lexicography and sociolinguistics.
Lexicography is, simply put, "the writing of dictionaries" (Language and Linguistics: The Key Concepts by R. L. Trask, second edition edited by Peter Stockwell. Routledge, 2007, page 150-151). It is a discipline with methods and tools that are very different from literary criticsm and literary theory. To give you an idea, I will quote that the glossary entry quoted above says about the development of lexicography in the last few decades:
In the last couple of decades, lexicography has been revolutionized by the introduction of corpus-based techniques, and modern dictionaries are now usually based upon huge corpora of English, from which words, forms, spellings, meanings and grammatical behaviour are extracted, thus allowing lexicographers to appeal directly to the observed facts of usage. Also noteworthy are the numerous innovations introduced by the COBUILD dictionaries and by other dictionaries specially written for advanced foreign learners of English. These describe words as they are observably used by the speech community, rather than as defined by the lexicographer alone or in prestigious texts.
The point of this quote is to show that lexicographical work is something vastly different from asking questions about meaning or interpretation in literary texts.
Sociolinguistics is "the branch of linguistics which studies the relation between language and society" (Language and Linguistics: The Key Concepts by R. L. Trask, second edition edited by Peter Stockwell. Routledge, 2007, page 264-265). Sociolinguistics studies phenomena such as diglossia, the effects of language contact, dialect geography and variation in languages between social groups (including male versus female speakers, old versus young, and linguistic differences between ethnic groups). This is another branch of linguistics that uses methods and tools that are unrelated to those of literary criticism.
Since both these linguistic disciplines are so far removed from the study of literature, attracting experts in these fields  would not significantly increase the community's ability to answer the questions that are already on topic here.
Based on the above, I argue that lexicography and sociolinguistics are off topic on Literature SE.
Does that mean that all questions about "dictionaries" are off topic? Not necessarily, since I see two types of exceptions:
- Dictionaries of literary terms. Since the meaning of literary terms is a valid topic on this site, we should also accept questions about dictionaries or glossaries of literary terms. (Such questions obviously need to conform to more general rules; for example, recommendation requests for dictionaries of literary terms would remain off topic.)
- Literary works that merely borrow the form of a dictionary with a totally different purpose than lexicography. Examples include The Devil's Dictionary by Ambrose Bierce and Dictionnaire des idées reçues by Gustave Flaubert. These works employ the dictionary form, not to explain the meaning of words, but to satirically expose certain ideas held by part of the population.
 I am skeptical about our chances of attracting experts in these fields. In the three and a half years of this site's existence, no professor of literature has become a regular contributor on this site.