Back in the 1980's, I saw a training video on this very subject. Some company came up with a devise quite similar to a sand blaster. Only this one used walnut shells instead. The intake manifold had to come off of the problem vehicle, and the crankshaft manually rotated to insure that the intake valve was fully closed. With the intake valve closed, the machine would set down in the intake port and blast away, as the vacuum portion of the tool sucked back the remains of the walnut shells and carbon build up.
So after one cleaned up the first cylinder, than on to the next one. Pretty slick tool if you ask me. The only downfall I saw with it was that it was time consuming, and you had to bloody make sure the intake valve was fully closed prior to using it. But it sure as hell beat having to tear off a cylinder head or 2 just to clean up the valves.