I have a cool piece of shareware on my hard disk which seems to handle this whole name/serial number thing quite well.
It works entirely on the humiliation-ware concept. Nobody wants someone else's name emblazoned on their $10 piece of software. How low can you get?
So anyway the program makes you send in for your super-secret registration code. This code is a really wierded-out checksum/encryption of your name.
So, for every name, there is a different secret code. The program won't let you have a mismatching code/serial number. It'll just say "Unregistered User".
Now, you could incorporate the user's name AND the serial number. Use a lot of XORs and tricky bit shifting to get a suitable ugly checksum and then convert it to a long (10 digits or so) decimal number.
Anyway, that's what FinderWindows seems to do. An excellent program, by the way. It adds a windows menu to the finder and other programs. Lets me keep 20 or so windows open on my screen and not lose a single one.
