How much work is it getting Moodle to accept OpenAthens as a source of SSO authentication/id management?
Most places seem to use Shibboleth - is OpenAthens a lot more troublesome?
I wouldn't say its more troublesome, but then there are inherent advantages of using Shibboleth or CAS over OpenAthens. There's an article at JISC with their views as to what advantages Shibboleth has over OpenAthens. Also OpenAthens by itself isn't a product, rather a grouping of products offered by Eduserv (Classic Athens and Athens DA for example). They also offer integration with Shibboleth
However if you did want to use OpenAthens for a single sign on, I believe OpenAthens is compatible with SAML.
I have to admit that I have fairly limited knowledge of working with OpenAthens. I only did a brief amount of work with it before switching to SAML and Metalib. So I might be wrong about a few things here, but I am sure some of our more learned colleagues will be able to chime in and correct me where I am wrong.