you'll be happy to hear it's a piece of piss and you're totally going to be able to do it. unfortunately, there are some hurdles you'll have to overcome.
first of all, the tbx tone control can be bought seperately, they even make an "eric clapton kit," but guitarelectronics.com is out of stock on both of them. at least you know they're sold though, some carefull hunting will yield results elsewhere:
http://www.guitarelectronics.com/product/PAEC
also, many other companies make a circuit board for installation in the guitar which will give you teh claptondtz, and use your existing tone control.
http://www.gtrheaven.com/emg_circuits.htm
http://www.guitar-mod.com/rg_active.html
i guess the biggest problem would be finding a place for the 9v battery. you may have to route. lucky for you, the jag-stang has all that extra room on its ass.