Either carving in, or branding on (not sure how to get that effect, but sounds like something Korn Orks would enjoy) sounds like a good idea. With all of the bits you are adding to the bikes, you may not want to add very many scars on top of the tattoos, but that might look excellent for the troops on the ground. Maybe something as cliche as one Ork having ripped the arm off of the Ork next to him and is now wielding the bloody stump as a club, while the owner of the arm is just using his bloody stump... as a club. You get the idea.