herokudns with custom domain: No such app
I'll answer my own question: according to Heroku's documentation the *.herokudns addresses are not browsable. Instead, Heroku uses the 'Host' header from the client to ensure the correct application responds to a request.
The problem here was that the Host header is coming with 'www' from Namecheap (since I made that my canonical name -CNAME-) but I didn't give the 'www' to Heroku so the solution is to add the custom domain in Heroku with 'www'.