Single Sign on for Customer Portal

Single Sign on for Customer Portal

I'm building an application and would like to use Zoho Subscriptions but digging through the docs have not yet seen a way to accomplish single sign on between the user accounts in my application and the Zoho Subscriptions Customer Portal for Customer Contact Persons.

Is there any way to do one of the following to accomplish single sign on between my application and the Customer Portal?

* Send Zoho Subscriptions a JSON token from my application, like Zendesk does when a user in the app attempts to visit the Customer Portal?
* Have the Customer Portal be the primary login location (user/pass managed by Zoho Subscription) and then have them redirected to my app with an authorization token
* Block password changes by the Contact Person in the Customer Portal and use the Zoho Subscription REST api to update the customer's password whenever it is changed in my application
* Any other reasonably secure account sharing mechanism that is automated