After messing around, this is what I know. The Touchstone backs put out a 5VDC charge, probably to match USB charging at around 500mA, probably around 50% effeciency. The spec that I can get without signing up, shows Qi is 5W at up to 70% efficiency. So that would put the numbers in the same ballpark of 5VDC and around 700mA max, within the USB spec, and likely closer to 500mA regulated. I would guess that the differences between the two specs is primarily the turns ratio, which is why placing a Lumia, w/charging back, on a Touchstone will indicate charging, but probably doesn't pull enough current to actually charge. I'm betting a Touchstone back coil will work with the Lumia, but the question is what contacts need to be hooked up to.
The Lumia has 4 contacts, instead of two, for the wireless charging. I suspect that the "extra" contacts are for either 1) current (2 smaller wires to handle the current of one larger one) or 2) for feedback for regulators. They could also be dummies, or have some other communication use for communicating with charging bases like the new HP stuff was going to do with the new version of Touchstones. What I would need is to see what the voltages are, pin to pin, for the 4 contacts of a Qi wireless Lumia back when it is placed on a Qi wireless charging base, sans the phone. This way, it would be easy to see what voltages it supplies - the rest is easy. So if anyone has one, and wants to put the information out there, I'll use it, and verify it, with my Lumia 822 I got off ebay (I'm still getting a free one from VZW at the end of the month, so I can experiment a little). I've also found out that the backs for the Pre's have 4 metal discs that are use to hold the phones to the magnets on the Touchstones. The 4 discs are place to where 4 corners where they would orient around the Touchstone, like a circle, allowing for the centering and multiple orientations. You need more than these 4 discs with the white Lumia because of the slickness of the back. I suspect the matte finishes will stay on, and I suspect this is why all of the Touchstone backs were matte. If not, simply adding 4 more discs, in alignment with the circular magnet of the Touchstone, will likely get it done, slick back or not.
Whew!!