using pbo doesn't make sense in your case since you already have the pixels in ram. the videograbber class (and the camera driver itself) grabs the pixel data from the camera and puts it in ram you can then send those pixels to a different thread and do the analisys there. until then the GPU hasn't been involved at all yet.
ofVideoGrabber also gets those pixels and puts them in an ofTexture (memory in the graphics card) so it's easy to draw the video.
what you have now goes something liek:
camera -> ram -> cpu
what you are trying to do by using pbo would be something like:
camera -> ram -> video memory -> ram -> cpu
which is way less efficient.
PBOs are meant to be used when you need to upload or download pixel info to or from the graphics card but in this case the pixels are already in ram.