Maybe I'm missing something here, but what does a concrete slab have to do with ground rods? Why can't you just come out the back of the disconnect with an LB (assuming its inside the structure) go down with a PVC pipe on the exterior of the structure into the dirt and connect to one or two rods?
You could try to run a GEC in the feeder conduit all the way back to the sourcing structure, but seems kind of expensive to run that long of a GEC and it won't be very effective. You might even be able to use the feeder EGC or neutral as the GEC as long as its #4 copper or larger. The inspectors don't seem to like a ground electrode system to be too far from the structure its supposed to protect, but I see no GEC length limitations in the code.
