[Geotiff] User defined datum: Transformation parameters

[Christian Zietz]

Hi,

as far as I know there exists no official way to store transformation
parameters (e.g. for a 7-parameter Helmert transformation) for a user
defined datum in a GeoTIFF.

So, before I go ahead and implement a solution using a private key ID:
Did someone do this before? Is there an unofficial specification I can
adhere to?

Christian