I'll take a stab at it--generally GPS position cannot be used in non-WGS-84 locations for terminal or approach applications. There are too many differences between WGS-84 and other datums to be safe. The ground-based navaids and ground features will not be where the FMS, used GPS positional information will not be where they are are. In the early days of GPS ops, there were numerous cases of airliners nearly hitting terrain due to these positional differences, esp. in South America. The Russian Federation is very definitely non-WGS-84, which is a problem with the future use of GLONASS.
Now, EGPWS data bases are compliant with WGS-84, so EGPWS can be used regardless of local datum.
How an operator would survey for these differences is beyond my knowledge. I suspect it would require a FMS system that would allow for the entry of different datums. This is feasible, as Collins did it for FMS-800 used by the US military. We had about 40 different datums and could program the FMS to use any of them. No civil system is equipped this way to my knowledge; it would create the possibility for all kinds of errors.
GF