This is just a preview since I just installed this yesterday. So far, so good. Also, to give credit where credit is due, this was inspired by the Jerry Donahue wiring. The implementation, however, is mine.
Here's a typical Tele wiring:
The blue box is a 3-way. Could be any kind. I'm showing the tone control in an odd position to make it easy to see the relative position of the components in the next pic.
Here's the mod:
In my preliminary mod, I'm using a 10k trim-pot for the resistor until I can determine the exact value that works best. I started off with it set at around 6.2k, which seems to work great. It's a .001uf cap.
Notice that when the 3-way is in the neck/bridge, or bridge alone position, that the switch effectively "shorts out" the added components. They are out of the circuit, and you have completely conventional wiring.
Here's with the 3-way switched to neck-only:
Notice now that the tone control doesn't just passively roll-off highs, but actually "borrows" a controlled amount of the highs from the bridge. Its like we have treble boost and cut, but instead of boosting the highs of the neck pup alone, we "borrow" those highs from the bridge. It not only adds a bit of "sparkle", but also "air" and "richness". We're adding in signal from a different source.
Thats the crux of it for now. Seems to work great, and you could do this mod with any pickup configuration. I'm going to try it on one of my dual-humbucker guitars also. I'll try to post a clip later, but I'm just getting ready to head out to my granddaughters b-day party.