You could try something like this:
EDIT: Made a mistake in my code. Updated.
// moves an object on the x/z plain, relative to the camera's y rotation
// obj = object id to move
// xAmt# = amount to move the object right/left
// zAmt# = amount to move the object forward/backward
function MoveObjRelativeToCamera(obj,xAmt#,zAmt#)
// store the original object rotation
ox# = GetObjectAngleX(obj)
oy# = GetObjectAngleY(obj)
oz# = GetObjectAngleZ(obj)
// set object's y rotation to match the camera's and zero out x/z rotation
SetObjectRotation(obj, 0, GetCameraAngleY(1), 0)
// perform move operation
MoveObjectLocalX(obj,xAmt#)
MoveObjectLocalZ(obj,zAmt#)
// set object's rotation back to original
SetObjectRotation(obj,ox#, oy#, oz#)
endfunction
