Brandon Sanderson writes people in quite often. Admittedly, he writes a HELL of a lot faster than George does. I've had a dozen or so friends written into the last couple of books of the Wheel of Time series.
I was hanging out with some friends (we run a convention together) hosting a room party when George walked in and hung out with us for a while. We even got him to snap some pictures with a cardboard cut out of Rand al'Thor.