My OBS never tested for GBS. Aparently it can change - you can be positive then you can be negative. I don't see the reason for intervention given it can be cleared up via antibiotics (exactly what OceanPrincess has said).
Also, if it is in your birthing plan, why wouldn't the hospital staff follow this?! Surely if they let you 'submit' a birthing plan, then they should follow it?! (maybe I am just naive)