Multitenancy with custom domain names

We have a multitenant application with custom subdomains using a wildcard SSL on Heroku. While this has been great for our clients they are now requesting the ability to add their own domain names.

Any suggestions on
1: How to handle routing of custom domains to client subdomains.
2: How to integrate this into Heroku dynamically.

Any notes or direction on implementation of this feature would be greatly appreciated!