I cut the cracks out of my dash, filled them with expanding foam, glassed over portions of the top, smoothed it all with bondo, and topcoated it with SEM interior paint. It looks great and is pretty durable (if you remind me I might be able to find pix).
The only problem was that I had the dash out of the car for so long that it relaxed it's shape a little. When I put it back in, it was too wide for the car and squeezing it to match the mounting holes made the new top surface warp a little bit around the cracks. Where it was totally smooth when I finished working on it, there are now a few small waves/ridges. I would recommend doing this work with the dash in a jig or in the car with the windshield removed so it fits correctly.
Looking good so far though.