Hey there! In a modern DNSOnly system, cPanel actually listens on the common ports instead of Apache, which is why you're able to get a 404 page at all by visiting the server's hostname. You can confirm this on your machine with this command:
and you'll see something similar to this:
Code:
# netstat -lnp | grep :80
tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN 1272/cpsrvd (SSL) -
There is currently no way to customize that page since the contents are generated directly from cpsrvd through a perl module, although that sounds like it would make for a good feature request if you'd like to submit one using the link in my signature.