If you can find out how MemberPress is determining which users should be able to access protected content on WordPress, then the same data can be used to restrict SSO logins to Discourse. To do that, you will need to add a conditional statement to the two functions in the second post of this topic: How to prevent some WP users from being able to login to Discourse. The conditional statement should return true if the user’s account has been suspended.
What you are wanting to accomplish is doable, but you may have to get some help from a WordPress developer to figure out what goes in the conditional statement in the two functions that I linked to.