IIRC, a ground rod has to be driven. Putting it in a hole and filling in around it would not meet code. However, I don't see why you could not drill a hole, fill it in, and then pound the rod in. Probably are going to need two rods anyway if you end up using rods.
I vote for a CEE as well unless there is some other issue going on like this is a cell tower or an antenna. if that were to be the case, the engineer would have plans for you showing exactly what to do.