First, I'll answer your other question, about converting 6v VW's to 12
volts. I recall the VW mechanics telling me how they left the 6v starter,
etc., in the car, so the engine would REALLY spin during the cranking. They
also applied the full 12 volts to the 6v ignition coil, during starting
only, to get a REALLY hot spark.

If the Hobbs meter draws current in short bursts, chances are you could use
the full 12 volts. If the wind-up is self limiting, the pulses would be
shorter than standard, and probably work perfectly.

If you want to be really safe, you could wire some resistance in series with
the meter. Use enough so the meter doesn't work, and then keep reducing it
until it sounds like it is pulsing normally. That would be a way to do it
if you don't have a meter, or if your meter won't deal with short pulses
(most of them won't.)