Woah.
Well, that would be interesting. We'd still suffer from effectively one volume control, right? And even if not, we'd still get the volume drop from the other pickups when the active one is engaged. At any rate, yeah, I don't think that exists.
But two of these: http://www.guitarelectronics.com/product/CPCS55/CTS-Stacked-Dual-Concentric-Audio-Pot-500K500K.html
And one of these: http://www.billsmusiconline.com/a25kx2.html
Would get me where I want I think. Maybe. I'm concerned that the degradation in sound quality of the passive pickups when connected to the low value pots will be worse than the "switch-like" behavior of the high value pots with the active pickup. EMG makes a preamp for passive pickups that will get them to work right with low value pots, but it's another mini toggle switch, and $60... No fun.
The more I discover about this the more I think I don't want to go this far with my re-wire. I will probably end up just wiring my second tone control to my bridge pickup, putting a treble bleed mod in, and just have no tone control on the EMG. We'll see. I'll play it as it is a little while longer and see how I feel about it. The other thought is to wire up my pups on the switch so position 3 is for the bridge pickup, that way I can use one or two passive pickups together, and the only combination of passive and active is position 4, which would be the bridge and middle. Position 5 would be the active pickup alone. Might lead to some interesting sounds, which is what I'm clearly in to.
Neck
Neck and Bridge
Bridge
Bridge and Middle (active)
Middle
With that setup and all dual concentric knobs (with low value pots for the Middle pup) I could probably be happy.