Predicament bondage? Tie the sub with the wand just out of reach - close enough that she can *just* about feel it tickle her pubic hair - and tied so that a little stretch can put it in the right place. However, make this stretch painful - weighted clamps on the nipples that put their full weight on when she moves, clover clamps on parts of the body tied to the wall that tighten when she pulls away from the wall, etc. Alternatively, you can make it so that she has to stand on tiptoes to get the sensation - very difficult to do for a long time. Or you can set it so that she can give herself pleasure freely but forbid her to cum until you command it. Set an appropriate punishment for cumming too soon (and she probably will...). This can be made even worse if you reverse the ideas above and make it painful NOT to be touching the wand.

You'll need somewhere secure (and safe) to tie the sub and somewhere secure to place the wand so that it does not move.