g&l's have this. the wire that goes to the volume pot from the switch goes to a tone pot instead. you will use the wiper and the lug that lands to the right if you look at it from the back and the lugs face up. one hot wire comes from the switch and another goes to the volume pot but it doesnt matter which goes to which lug you use on the tone pot. a capacitor (a good one, orange drop maybe, it is important for this type of control) goes from the wiper to the right lug. as you turn the knob clockwise it shorts the cap making it normal sounding, counter clockwise and you are in series with the cap cutting bass.
http://www.glguitars.com/schematics/Legacy_schematic_drawing.pdf
^directly from G&L, messy looking i know but it'll get you what you want.
you can use a resistor in parallel with the cap to limit the range.
option 2 is to get a Q-filter from bill lawrence, it normally works in a notch filter arrangement. in a notch filter a cap is series with an inductor to ground, the inductor being the Q-filter. capacitors pass highs but lows will see an impedance near infinite, so when shunted to ground the retain lows in the signal. inductors have near infinite impedence at high frequencies and so when shunted to ground you retain highs.. so the two wiring options using a Q-filter are to A) use a resistor in parallel to the cap as to limit the retention of the lows. 5-10k ohms works great. makes the guitar quack. can almost sound like a piezo system. or B) forgo the cap all together and run the Q-filter in series with a resistor atleast as big as the dc resistance of the pickup in place of a cap in a normal tone circuit. i haven't tried this but i expect it to cut the output quite a bit.
http://guitarsbyfender.yuku.com/topic/2992/t/Q-filter-What-does-it-do-exactly.html?page=2
^ these are some examples of posible circuits, even one that can be selected as a normal high cut via a switch.
