Slim, I think you may be confusing the Nero serial number with the drive's serial number.
Think of it this way: the software from Ahead contains all the features of the full Nero product, but only certain features are actually activated depending on the Nero serial number you supply. When Nero starts, it looks at the serial number (of Nero, not the drive) and activates features accordingly. In an OEM serial number situation, it determines that it should only work with drives from one particular manufacturer, and it can tell which manufacturer is allowed based on the Nero serial number. Nero then scans the bus, looking for ID strings that match that manufacturer, and allows you to burn just to those drives.
One consequence of this is, as you said, that if you use one of the handy utilities for flashing rebadged LiteOn drives to "true" LiteOn drives, your OEM serial number for Nero won't let you use the transformed drive because the ID string no longer matches.
One way you can get around this is by purchasing a full license from Nero for $19. Another way you can do it is by using a binary editor to modify the LiteOn firmware update file so that it retains the original ID string ("MEMOREX", in your case).
Be sure to check your private messages
, Slim. I sent a question about your firmware.