I could see Starfleet thinking "we'll have more ships than nine!" when giving the registries to the Enterprise
and the Columbia
, but correspondingly not thinking "we'll have more ships than 999!" when registering the Grissom
Then again, if people don't like the idea of NCC-0514, they can always say the Kelvin
is NCC-O514. No character on screen is likely to pronounce the "zero" or "ough" anyway, so the verbal form of registry will still be simple 514; whether one adds the TAS-style definer "O for Observation" would be a matter of personal preference, and perhaps George Kirk doesn't prefer to.