The only use of the proverbial 16-ton weight, that I know of, was the Salem witch trials where a man was sentenced to be "pressed" under a large slab. Given a sizable iron weight dropped from a fair height, it would be pretty darn quick (if that's the definition of humane they want to use). Pretty easy cleanup too, just hose the room down, and a fill a drum from the catch basin with the remains. ;-)
Of course what the founders had in mind with the term "cruel and unusual" had to do with justice that was in recipricol "just" with the crime. That is any form of execution for murder could not by definition be "cruel and unusual". OTOH, execution for petty theft is exactly what they meant by a "cruel and unusual" punishment.
Pressing was done by adding weights, not by sudden impact/crushing.