The bucket seats that they are referring to are the ones that completely wrap your head and give you the lateral protection for your head. With those lateral protectors you cannot clear the "blind sides" that a mirror can't do. Regular buckets without the laterals for your head and such like the ones on the GT3's are fine.
Also, remember that some GT3's come fully caged but DON'T come with the lateral support bars by the doors. I don't think that the cars from RSR or R4R come with those lateral door bars either but it would depend on who is interpreting the rules at the gate. Mainly they don't want a car that you have to contort yourself to get in running on the track because they are more considered full fledged track cars
Quote from Ezeedee regarding car mods:
"you're only done when you sell the car