Yes, this is now supported! Memberstack now allows you to use Memberstack as an OpenID Connect provider: https://docs.memberstack.com/hc/en-us/articles/8161104982043-Custom-SSO-Integrations. This will allow users to login to your Discourse site through Memberstack. If you configure your Discourse site so that OpenID Connect is the only available login option on the site, logging into Discourse from your Webflow pages will be a seamless process for your users. (Note: don’t remove the option to login to your Discourse site with a username/password until you’ve confirmed that OpenID Connect logins are working.)
For this to work you’ll need to have the Discourse OpenID Connect plugin installed on your Discourse site. Details about configuring that plugin are here: Discourse OpenID Connect.
I recommend watching the video in the Memberstack documentation that I linked to before attempting to configure OpenID Connect logins for your Discourse site. It takes you through the process of configuring Memberstack OpenID Connect logins to work with the test site at https://openidconnect.net/. After you’ve done that, setting up OpenID Connect authentication for Discourse should be a straightforward process.
If there are any existing Discourse sites that have configured OpenID Connect authentication with Memberstack, it would be great to hear from them.