They've had Ken Block on twice outside a SIARPC context.
Also, if he or Tanner appeared and did a lap, what board would they go on? They can't on the normal board; they're racing drivers. But they can't go on the F1 board either; they're rally drivers. It's either expand the F1 board to include other racing drivers, or start a new board for other racing drivers.
Super late reply is super late, but I would love to see Ken and Tanner go head to head in a non-rally context as SIARPC. I wonder if rally drivers would take the same lines as F1 drivers?