My favorite current "Wah" set up is by using a 250k Tone pot with a .1 cap.
You could wire in a 250k Push/Pull so that when the pot is pushed "in" its the wah tone using a .1 cap, and "out" is the standard tone control using a .22 or whatever cap you use currently.
http://fralinpickups.com/images/teleschem.jpg