Hi Simon,
I believe you should wait for a response from Mr. Tore or Maciek... I was just thinking of a drawing in Skizzenbuch where a valve for the header tank or sea water selection was located at the bottom of the saddle tanks. If they had to change the selection, then the valve stem would need to come through the pressure hull to change from the fuel oil mode to the ballast mode.
Skizzenbuch -
"The compensating water pipes goes from the header tank to a small chamber (about 1.5 % of the tank volume) at the bottom of the tank, from this chamber a short pipe goes into the tank bottom and an equalizing pipe directly to the sea. The two connections are controlled by a double seated selector valve operated from the casing. The valve has only two positions, either to the fuel tank or direct to the sea. When in the fuel oil tank configuration the Kingstons are shut and the selector valve open to the fuel tank and thus shut to the sea."
Regards,
Don_