Don't just put GPS unit on the extremities, put IRS units there too. I know its hardly a comparison to what will be needed, but imagine an iPhone on each wingtip, the nose and the tail. The accelerometer data is not for navigation, so 0.3 ANP etc isnt relevant. It's for attitude info only, realtime. The more accurate the GPS the better, and each system would complement the others and make it even more accurate.
Why in gods name would we need something as complicated and failure prone as that when we have three excellent laser gyro driven IRS already which provide attitude information? And a fourth attitude source in the backup instrument?
Attitude information is not the problem, flying the correct attitude is.
And differential gps is all nice and dandy, but it usually requires ground equipment in a short range. Google GBAS and have a read of a real world approved and certified differential gps application in aviation.