[Proj] grid shifting

[cp]

Firts, excuse me for my poor english.

Just a question to see if I have understood the mechanism of grid shifting 
in proj lib....
The grid shifting is a correction of phi and lambda in a particular point on 
the ellipsoid : this corection is made by a matrix which is unique in a 
"step" of longitude and latitude. Isn't it ?

So datum transformation can be schematized as :

Plane Coordinates A => Geodetic Coordinates A => Grid Shifting => Geodetic 
coordinates B => Plane Coordinates B


So it is not possible (except by modifying code) to make a correction for 
these cases :

1) We have a grid of correction that represents differents values of 3D 
shifts for geocentric coordinates
like this :
Plane Coordinates A => Geotetic coordinate A => Geocentric cartesian 
coordinates A => Grid shifting => Geocentric cartesian coordinates B => 
Plane Coordinates B

2) We have a grid of correction that represents differents values of plane 
shifts
like this
Plane Coordinates A => Grid shifting (matrix : grid is based on planes 
coordinates and no more on geodesic coordinates) => Plane Coordinates B

Isn't it ?

We have to do this because of increasing accuracy of transformation beetween 
systems ( the grids could be experimental calculations)

thanks a lot for reply