This information seems all the more important as the changing face of publishing means I am pitting my library against my professional association for the advancement of scholarship.

That's the answer.

Looks like MLA runs their journals in-house, i.e. they don't subcontract out to a mega-house, like NCA does with Taylor and Francis:

http://www.mlajournals.org/page/about_mla_journals

This affords MLA the flexibility they need to adopt the author-owns copyright policy. As far as access goes, great policy for authors and readers. It will probably cost MLA some non-member subscription revenue, but this is likely not a big cash source for the organization, which almost surely generates the bulk of its income from membership dues.

Slight tweak on a good question that drove this thread - what does NCA's contract with Taylor and Francis do for the organization (how much $$, circulation, academic cachet), and authors (copyediting, typesetting, placement in search engines)? And what are the costs in terms of limited access that stem from the alliance with mega-publishers?