Out of interest, will ATCUs have a link to the database of Mode S codes, providing them with an instant lookup of the aircraft identification?
That shouldn't be necessary. To use a computer analogy, Mode S equipped aircraft have both a hardware address like a MAC address, and a user configurable callsign like an IP address that can be set before each flight.
The database you describe links the reg to the hardware address. But Mode S transponders also transmit the callsign as one of the downloadable parameters, just like a computer puts the source address in each packet -- for most GA aircraft it can be stored in flash memory and never changed as it's the registration. Commercial crews will have to set it for their callsign (or have a clever FMS, I guess).