I think people have found out about this already, but in case they haven't gold digger AI works if you set waypoints underground for a miner to follow. He will dig towards the waypoints.
The only catch to this is that if the waypoint is too far he will try to take an above-ground route to it. So put them close together and you can have a reasonably functional gold digger AI.

Thanks.

Setting them to gold dig and watching them as they dig up can be frustrating.

I wonder how hard it would be to detect gold with rays.
From there it's trivial to set waypoints.