@Felli Yeah, you can implement a single sign on but it'd take awhile to create a fully customized single sign on as it will take a lot of file editing and custom coding to string them all together. (This goes for customly made websites, not wordpress websites as that would take a lot longer.)
I've been thinking that way. But that's more like hacking into the system, it will introduce too many modifications and would prevent future updating.
I'm thinking about a plugin natively in the eso forum system so that the admin will just need to create a config file to string everything up.
As for the eso part, it will behave as a Service Provider. I got those basically from here: http://vanillaforums.com/features/single-sign-on