We are trying to configure Zendesk to authenticate against SalesForce Community User accounts using SSO.  So far, we've successfully configured Zendesk to authenticate against SalesForce internal user accounts, but not Community User accounts.

The current setup is relatively straightforward...
We configured a Domain per these docs: https://help.salesforce.com/apex/HTViewHelpDoc?id=service_provider_prerequisites.htm&language=en_US
Single Sign-on with SalesForce as the Identity Provider and a Connected App for Zendesk, in a similar fashion to these docs: https://developer.salesforce.com/page/Configuring-SAML-SSO-to-ZenDesk

Amongst the many things I've tried, I tried changing the "Identity Provider Login URL" on the SAML Single Sign-On Setting page to point to our Community custom login page, which at least redirects the user trying to login to Zendesk to the right login page.  However, the SAML assertion doesn't work and the user is not redirected back to Zendesk after login.

I have found no documentation or articles on using SSO authenticating against Community user accounts, so any direction from this community would be greatly appreciated!