Unused coil in coiltap as inductor 
 

I just "invented" this by accident, no doubt it's old news. The unused coil ends up in series with the tone control and acts as a variable mid-cut. Normal humbucker operation is unaffected.
It gives a useful twangy tone.

http://www.guitarkit.com/coiltap.gif


  Some pickup inductance stats here:
http://www.buildyourguitar.com/resources/lemme/
I assume the capacitor on the tone pot can be varied usefully, a DPDT switch could change the cap as you coiltap. I'm not sure about the math involved in working out what the values should be. Anyway, it's a something for nothing deal, and an excuse for tinkering .

http://www.guitarkit.com/coiltap2.gif
 
Turning down the tone knob moves the peak up in frequency, and down in amplitude. There is a mid cut.
 
Reducing the tone cap can give different frequency responses.
 
Ignoring cables and amp loading.
 
(See http://www.buildyourguitar.com/resources/lemme/ for theory)

The volume drop is slightly less than volume drop of the coil tap. So it's a bit like tapping a tapped coil.
Mid-song on a Les Paul might be a switch from tapped bridge "Rhythm" to neck "Lead".
 
The gist is that a normal tone control lowers the frequency of the reasonant peak of the pickup, while this one increases it, and flattens the peak. So the high midrange honk of a tapped humbucker becomes a mild low treble honk.
 
It doesn't increase or reduce hum.
 
Bill Lawrence uses a resistor in parallel with the tone control cap. "when you turn the control to zero and you use hi-fi speakers, you can duplicate the sound of an acoustic guitar". That is what the coil tap mod on the bridge reminds me of. "With the controls on 3, you can get out of the bridge pickup, the #4 position sound of a Strat". Halfway there, I think.
 
I think the magnetic core affects inductance, I forget.
 
You have an Varitone stompbox as I recall, so very-tone dog looks like a good project, a humbucking pair of Mouser #42TL021, a rotary switch o' caps and resistors and a booster. Or a Wild Mouse.

Hi Steve!
Wire the inductors in series or parallel (OK, series/parallel switch) and turn one upside down. In theory, I haven't actually done this.
 
Bill Lawrence sells a $20 noiseless inductor for this kind of thing. And he seems to know what he's doing. http://www.billlawrence.com/Pages/Pickup_Window/Q-Filter.htm
 
Circuitmaker for the graphs. It's a free download from http://www.microcode.com/ if you don't already have it.
 
The L6 tone control could be a treble bleeder cap across pickup one perhaps, I'm not sure. You could use an unused entire pickup as an inductor, why not.
 
I was thinking of using a 5-pole PRS style rotary switch to do something clever, I may have to wait for cleverness to arrive. This guy has plenty of ideas: http://www5.ocn.ne.jp/~dgb/index_e.htm