For reference I did eventually solve this without using ref-array.
The trick/hack is to know that in C, an 'array' is pretty much the same as a pointer to the first element. So in ffi, we simply pass a pointer to the first element, and be very careful to not overstep our bounds.
var pface_type = ref.refType(face_type);
var mylib = ffi.Library('lib/libN', {
'GetFaces' : [ 'int', [ ref.refType('int'), pface_type ] ]
});
var ppface = ref.alloc(pface_type);
var face_count = ref.alloc('int');
result = mylib.GetFaces(face_count, ppface);
var faces = [];
var count = faceCount.deref();
if(count > 0)
{
var face_array = ppface.readPointer(0, count * face_type.size);
var i = 0;
for(i = 0; i < count; i++)
{
var face = ref.get(face_array, i * face_type.size, face_type)
console.log("X:" + face.rect.x + " Y:" + face.rect.y);
}
}