How to use SSL/HTTPS with a Heroku app that needs many different (wildcard) domain names?

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http://serverfault.com – I'm hosting a SaaS application in Heroku and my service offers user groups a possibility to get their own subdomain address for the group. Say, my service is www.example.com and a user group can get our_group_name.example.com as direct domain name for their group. This all works and HTTPS is possible as we have *.example.com wildcard certificate in use with Heroku's Endpoint SSL (in Cedar stack). Now I want to add a possibility to get a totally custom domain for a group. For example www.our_group_name.com should point to the same app and work with HTTPS. Heroku's endpoint SSL seems to allow (HowTos)