If you check out the gear section, AtomicMassUnit actually did this and described it in detail in the thread. The thread's title was '7-sound strat'. What he did was remove the wire from the toggle switch to the middle pickup and attached an On/Off switch to it instead. He then rewired the toggle switch so that the selections were Neck, Neck/Bridge, Bridge and Off. So then you can turn the middle pickup at any time, and so you can have all pickups on at once or you can have a telecaster sound with the neck and bridge pickups selected.

The thread goes into more detail. A good site to check out re: guitar modifications is http://www.guitarnuts.com.