I would not use Seafoam in a diesel.
Seafoam is a solvent never intended to be used in a diesel. Introducing it into the fuel system would most certainly reduce the lubricity of the fuel, which would put the injection pump and injectors at risk.
Introducing it into a vacuum line as described in the directions on the can (directions for gassers) poses two problems.
1. Engine run away - Seafoam is highly combustible. In an engine such as a diesel with no throttle plate, this results in unrestricted fuel and air.
2. Hydrolocking - diesels are high compression engines, thus there is minimal combustion chamber volume. Literally an ounce of any liquid in the combustion chamber of a TDI would result in extensive damage.
Seafoam is a relativly effective product when used carefully in accordance with the directions. I've used in my gasser pickup, but I'd NEVER USE IT IN A TDI.