I'm a big fan of open-access publishing: Philosophers' Imprint, NDPR, and the Stanford Encyclopedia of Philosophy are all wonderful resources. I only wish that we could add to this list a respected source for open-access philosophy textbooks. One can always self-publish, of course, but then readers lack the quality-control assurance that selective publishers provide, and the author won't get as much professional credit for their efforts (e.g. at tenure review time). I can see a few possible solutions:
(1) Some university (like Michigan, Notre Dame, and Stanford, above) could provide the necessary infrastructure, establishing a rigorous review process and publishing approved books -- both online and with 'print on demand' availability. (I wonder whether Michigan's Scholarly Publishing Office is already in a position to provide such a service for philosophers?)
(2) Alternatively, one could go the Holbo route and convince your traditional publisher to let you retain the "e-rights" to make the book freely available online (perhaps in some 'closed', e.g. print-disabled, format).
(3) An intermediate option would be to publish with someone like Flat World Knowledge: a for-profit publisher that provides free (but I think not fully 'open') online access and affordable printed books. However, they haven't yet published any philosophy books, and I'm not sure how rigorous their review process is, so this risks being regarded as a bare step above self-publishing.