What did you find out? I would love to know if it would work or not.
Venture didn't provide the details of why it won't work, but anyway MB drives the tach from a crank sensor with some electronics. It can be made to work with a lot of effort, but not simply off the alternator.
I found a tach that DOES work off the alternator, it was a from a boat with a BMW diesel marine engine with a similar alternator-based system. It has a full range of adjustment for fine tuning so I can set a baseline for the engine speed and once I do that, it should be pretty accurate.
I will post pictures when I get something working.
Volvo 760 D24T cars have tachs that work from the W terminal.