what if you had a method of turning the hack saw blade through 90 degrees so you can cut horizontally. A standard blade is 12" long, you need some movement on that say 3" total travel. That means you could cut almost 9" of rock.

The feed method in the horizontal would be a trolley, with a big weight on a wire and pulley over a table edge.

There are also much longer power hacksaw blades available. I have not found an 18" blade suitable for stone yet, but I'm still looking.