Since returning from the first trip we decided to fit fuel line heaters to the 190d:-
http://www.carbon-neutral-car.com/2007/11/fitted-injector-line-heaters.html
For the second trip which was in the 300td a stewing pot was added for cooking on the move. Its basically a pressure cooker, mounted tightly to the side of the block above the hot exhaust manifold. A shield was added to reduce the cold airflow which seemed to be keeping it cold.
http://www.carbon-neutral-car.com/2008/08/stewing-in-croatia.html
Modifications made for the most recent trip were mainly about comfort and increasing range. They include a 2nd battery, a 2nd fuel tank and better lighting.
http://www.carbon-neutral-car.com/2009/06/modifications-continue.html