Ben; Think of tipplets as extending the wing span. They make the wing tip area more efficient, and for a wing with dihedral in place of ailerons, or a polyhedral wing, the tip area is going to become more efficient (more lift/less drag), and therefore the dihedral effect will be enhanced.

I've never tried tipplets on a rudder-only design, but they should increase roll stability if not roll rate, much as adding another rib bay to the wing tips.