Sure, but formal rules are not equivalent to a prescriptive grammar. AAVE has formal, consistent rules, described by linguists. You can make mistakes in AAVE just as in standard English (see: "African American Vernacular English Is Not Standard English with Mistakes" https://web.stanford.edu/~zwicky/aave-is-not-se-with-mistake... )
like most languages in history, AAVE has not gone through a process of codification and standardization to formalize it
I'm not sure what you're getting at - prescriptive grammars of the codified form you're describing are the products of their political and economic circumstances. There isn't some Hegelian trajectory of linguistic validity, where all variants aspire towards legalism.