Good question as I cannot find any reference to the originality of the Staircase Prop in any of my books or notes.
The first touring illusionist most likely to use the Staircase Prop is Lafayette (Sigmund Neuberger, 1872-1911, killed in a theater fire at Edinburgh, Scotland) in his illusion Lion's Bride (still used today). The illusion probably fooled most magician's of that day as others were using, at that time, a somewhat similar method in the illusion The Cannon and Crystal Box. (But the Staircase Prop may go as far back as the mid-1800's.)
As to D.C.'s use of the Staircase(s) (originally presented as "Walking Through Eight Laser-welded Steel Plates" designed for stage by Don Wayne) while doing the illusion at the Great China Wall made the illusion much more stronger (and the Staircase Props seem more subtle- good call Mop!) than on stage.
The genius of the Staircase Prop (almost always used to "steal away" a thin person in a transformation effect) is its "need to be there" use. i.e. the assistant has to use SOMETHING meaningful to climb onto the elevated platform or box, hence the staircase, which in turn is of course taken away (seemingly alone) to continue with the illusion presentation. (The one drawback in days' old being 1 or 2 asssitants usually wheeled on the staircase and then 3 or 4 assistants wheeled off the staircase giving perceptive viewers a not-so-subtle clue as to methodology!)
In the present day Staircase Props have become extremely thin (due to superior building materials and a higher angle) and the use of smooth-as-glass wheels to become very deceptive to unaware viewers. (But the use of the Staircase Prop in ANY illusion has been shot to hell due to our notorious sellout magi, Valentino, who unwittingly tipped the Staircase Prop for all to see behind his infamous mask a few years back.)
Today's cutting-edge illusionists, such as D.C., have forgone the versatile use of the Staircase Prop and have done an incredible job of getting around this seemingly major set-back with new and innovative principles of stage magic.