Configuring Aegir to Set Custom File Paths
The answer is to mv siltes/firstsite.com/files to /var/nasmount, then create a symbolic link to it.
(from inside your sites/firstsites.com directory,  mkdir /var/nasmount/firstsite.com, mv ./files /var/nasmount/firstsite.com, ln -s /var/nasmount/firstsite.com/files files).

I would put this in a script so this becomes a drush command, then make it run at the end of the aegir create a site process.

I'm not an expert of where to put this script and/or how to create it.