The only way you could do it is by having a custom number set where 2 numbers are changed to be G and P Example: G-P-3-4-5-6-7-8-9-0 ( So in this case 1 and 2 are replaced by G and P, so users would not be able to run numbers that have a 1 or 2 in it. ) Same goes for an O plate, you would have to run O-2-3-4-5-6-7-8-9-0.
Note, 1 and 2 don't have to be the numbers that are replaced, you could find the least common number entered and change that specific number BUT you have to ask the mod creator for permission to change stuff on their mods as it is frowned upon to change other peoples work around.
D Grade - 800-1199 C Grade - 1200 - 1599 B Grade - 1600 - 1999 A Grade - 2000+
Is already the system implemented in KRP and the servers can be restricted to those license types only.
I don't see the point in adding in any other system/fixing the current one while the game isn't as popular as it once was. + The current one isn't even broken, it just requires 10 or so races to get given a driver rating.