I am under the impression that the rid and access card numbers are "hard coded" into the physical chips on the card and Receiver, and can't be changed without physically changing their chips, that's why they in part can't be spoofed is my understanding. There is no way to do that with an account number, as those have to change when DVRs are activated and deactivated. This is my thought process on this. I don't know if that's the case, but it was explained to me that way long ago and it did make some sense at the time, although I don't remember all the reasoning behind it anymore, just the overall concept.
Hard coded or not, it's just a number. The access card can be spoofed and has in the past. I don't see why you can't spoof a RID either, it's just a number. Not sure why you think it needs to be a "hard coded" number though, a number is a number. They could encrypt the recordings on any drive with any passphrase, why not use the account number?