I just went and acted like I was registering a new one and was able to enter all fields and could click the register button. (using I.E. on Windows 7)
Updated: I just went on thru Safari on my Mac and was able to put a false serial number in and register it. I then went and tried to put in an ESN after that and it allowed me to start putting in the ESN on the false scanner. I did not try to complete it because I don't want the Uniden God's mad at me.