With a multisite WordPress setup, it is possible to enable the plugin for all subsites on the network. Doing this will allow all subsites to connect with a single forum.
Another thing that could be possible in the future would be for WordPress sites that are not in a multisite setup to connect to a single forum. This could be done by using Discourse individual user API keys, instead of the All Users API key. With this setup, it would not be possible to use SSO, but the owner of the site would be able to publish to the Discourse forum to categories that they had access to.