Yes you can. Last weekend for the WTCC they prevented cars going onto the grass due to the very wet conditions, so we had to park on the rallycross track behind the barriers.
They charged £2.00, I seem to recall it normally being £5.00 for southbank parking.
Also note that the entrance for southbank is changed. It's not through the main entrance anymore, you go about 200yds further south on the A20 for the southbank parking entrance.