I've just custom CNAME for Idea Portal, but I can't custom CNAME to Aha! main
Thank you for the note. As you mentioned, we do provide the ability to support a custom CNAME for Ideas portals to allow you to extend your brand to a customer facing portal.
However, are unlikely to make updates around providing custom CNAME capabilities to primary application. However, as you likely know, you are able set your own account name so that it appears in the URL subdomain.