p2p is the way to go. ftp means constant server space and bandwidth and that means money. And there is NO way to tell wheter or not someone is a registered user unless you ask for people's keyfiles which will be ridicilous but I know someone who did that once. I'm a regged user myself but I registered with my real name and address, it's inside the keyfile and any attempt to prove my registration will reveal this information. Wepl wouldn't know I'm "burseg" for instance, he knows my real name.
There are already some games like moonstone that don't work without registered whdload. Perhaps a check tool can be made that works similarly, only with a registered whdload and outputs an encrypted information file that works like a certificate but without personal information contained in it but then again an ftp will be required to control the downloads and bandwidth isn't free.