Actually, the closer that you put your control surfaces to the tip, the LESS efficient they will be (less lift per unit area).
The reason for this is due to the 3D flowfield at the wing tip. If you consider the lift on a 3D wing (sorry, i don't have a figure to attach), the lift goes to zero at the tip.
Your getting air flowing from the bottom of the wing spilling onto the top of the wing... killing the pressure differential that allows for lift.
What does this mean for your tip control surface? Your going to need a lot of area, and a lot of servo to move that area, for the same effectiveness as a conventional aileron.
DON'T let this quash your idea! Your definitely going to want to use a cut-off tip, or better yet, an end plate. Try it! Why not!
Ken -
www.litkoaero.com