Mega + SIM808 GPRS GSM GPS IPX-SMA module power on from code
Looking at the pictures it looks like it should be possible to take the two exposed connections of the switch and add flying leads to them and run them back to the microprocessor to allow it to turn them on and off.  The possible problem is the current that has to run through the switch.  

You will need to investigate, but it looks like it could be done like that if you can't find a better solution.