What happens if you pluck a hair thats rooted exactly in the center of and under a scab?

You pluck the hair. That's it.

If the hair was still in place after whatever injury caused the scab, then you pluck it, and the root comes out, it means the follicle was intact.

That in turn means that, assuming the motion doesn't remove pieces of the scab, that it's just like plucking any other hair.

That's not uncommon at the edges of scabs. People will pull away a scab, and the hair gets pulled out because it was partially buried in the scab. But you'll also see hairs poking through scabs at times.

That's it. Hair comes out, end of story.

The hair follicle may be the root cause of the scab in the first place. Acne, as a result of an infected follicle, may appear to heal with a scab but the infected material may still be in-situ.

As you know, the hair itself is dead past the growing root.

So plucking the dead hair will do nothing, unless you are lucky enough to take all the infected material with it.