Perhaps a good mortar and pestle can do the trick.
This would be closely related to your hammer and divot-in-concrete method, especially if using a good strong mortar and pestle (mine is heavy granite). However, the benefits of this are that it is easier to handle and move (you can keep it at a workable height), you can control the force more precisely (several lighter taps to fracture, then tease the shell apart, mean less chance of the kernel breaking), the nut is more contained in a deeper bowl, and so won't flee if the strike is off-center (again privileging several lighter taps, preferably at different points on the shell).
In case I was unclear, the emphasis is on several sharp controlled taps to fracture the shell, not great strikes which might injure the kernel, or more importantly, yourself. Let the weight and hardness of the pestle do the work. The force of the strike, and the nut, will be contained in the bowl of the mortar.
Method is basically drop a nut in the bowl, thwap at it a few times until you hear a crack, move on to the next. If it won't tease apart after, drop it back in for a few more thwacks. I've shelled different kinds of nuts this way, though not macadamias specifically, but I see no reason it should not work if your mortar and pestle are of a decent size and weight.