I have had nothing but frustrating conversations with tech help lines. It's sad how often they just have absolutely wrong information about their own products, much less an understanding about how other products work.
There are so many different permutations of ignitions and tachs, I don't know that there will ever be one adapter for all possibilities.
To answer a PM based on this thread, I'll post the circuit I ended up using. It only applies to the stock ignition and a Sunpro Minitach from 15 years ago. If the tach design has changed (as they have for that particular tach in the past), the circuit may not be valid.
I originally used 1k resistors, assuming they would be well within an acceptable range. Turns out the range of acceptable resistances was far narrower than I thought. Without using an oscilloscope, I don't think I would have come up with the right values.
Incidently, a Dyna S would require a totally different design from this one, due to the dwell differences.
for kohalikkalamees: