it isn't bad though? i do a similar effect to this which everybody loves - although it does take more practice to make it not "noticable"
If you learn the second deal, do the "shuffle" you did and start second dealing yourself, and ask them to say stop.
If however, you do want them to deal, i would recomend what katrie said, as in just "point" to the messy pile and do the move then. I have done that often when i...errrm...."help myself out" when playing poker

(do your move, and then let them cut the pack, and undo the move on top of the deck

)
but not a bad idea overall, a couple of tweaks here and there, and this could turn into a really astonishing trick!