DNS Alias in SSRS Subscriptions
In case it helps anyone else.  If you open Reporting Services Configuration Manager you can specific a URL for the Web Portal. If you use the DNS alias here this automatically sets the subscription emails to have the DNS alias.  The Web Service and Web Portal address have to match and make sure you have your Kerberos/NTLM settings configured properly or you'll get an error when you try to manage subscriptions from the server itself.