And then what? The bandwith would still be charged against host.name.com and that's exactly what we're trying to avoid.
The idea of strictly using the server's ip / hostname and then parsing that log file to check for the username would still be best imho. This way the user can fully take advantage of having the server's ip/hostname ~ feature (or the vhost, whichever you prefer) but no matter what it would be charged against his bandwith. I couldn't care less if he was doing it that way or using his own domain, whereas now I do care since I keep on noticing users which prefer to have big image files loaded via ~ and therefor reducing bandwith.
btw, you'd not only have to add that A record, you'd have to create the vhost in the user's httpd.conf as well right?