Hi Niklas,
Regarding Question 1:
What is the “all other software”-way of storing no-data in GeoTIFF?
Actually GeoTIFF has no means to indicate a certain value to be used as no-data.
That’s why we add the DIMAP header as additional metadata.
We could convert int8 bands to float and set all -128 to NaN. But I think you don’t want to have the data changed in this way.
If you have a good idea how we an tell other software which value is the no-data value, please tell us. We would be happy to consider this for the further development.
Regarding Question 2:
You need a library in python which can read GeoTIFF and gives you access to the metadata and then parse the store DIMAP xml.
Maybe the user @hollstein has found a way to do so in this thread.
cheers