If you have access to your DNS, you can add an A record for your domain name that points to the IP address for 'master . signage . me'.
Assume your domain name is 'domain.com'. Add an A record for subdomain.domain.com that points to IP address for 'master.signage.me'.
Now, instead of using newbielink:http://master.signage.me/MasterServer/signagestudio.aspx?mode=login&eri=nnnnnnn [nonactive]
use newbielink:http://subdomain.domain.com/MasterServer/signagestudio.aspx?mode=login&eri=nnnnnnn [nonactive]
This should effectively hide the 'master.signage.me' domain name from your users. The only downside is that if the eri is removed from the URL, users may still see the MediaSignage branding. I remember seeing something in the tutorial videos about the MediaSignage servers setting a cookie so that it remembers your users using your portal and prevents them from seeing the MediaSignage branding. Not sure if it will work for this or not, however.