Aren't there active mid-boost circuits you can add to a guitar, that run on an 9V battery? EMG makes one, but I don't know if you can use it with passive pickups.
http://www.emginc.com/products/index/144/146/4
I think this would be better:
http://www.starrlabs.com/products/guitar-electronics/the-spectrum-1-2-stage-midrange-eq