Ive created a program that emulates a MTK gps from coords received from Xplanes.
So you connect a FTDI cable to the gps com port on the mega, and the program sends MTK binary messages to your APM. Its included in the latest ArudpilotMegaPlanner 0.4.6
Com9 being the usb on the mega
Com10 being a ftdi cable wired to the breakout on the gps port.
Config setup – MTK as the gps parser
Ground control station being 6
Commented out read_airspeed and read_ARHS
Changed gcs_protocol == 6
And attached is the modified GCS_Xplane_mine.pde