Never heard of it, but it sounds like it should work.
Most diesel tachs work in one of two ways: Counting pulses from the alternator, or counting pulses from the teeth on a gear ring on the camshaft. So, pulses is pulses and this one is counting pulses from the fuel injectors.
Should work just as well, clever idea.
You probably an get a used Yanmar gauge from Sailorman (Ft. Laud) or some of the other "used boat parts" places, with a little patience. Then it is just a question of whether you need a sender, or a connection to the alternator, and you probably can fix up the original system too.