You should be able to trigger a 12 volt tach with a six volt ignition system as long as you are supplying the tach power supply with 12 volts. You could run a 12V boost converter, should only need a couple amps. You could convert the back light bulb to a 6v bulb to wire to your instrument panel. The tach should trigger from the points grounding the sensor wire and should not be sensitive to the amplitude on the trigger wire, which is only about 3-4 volts even on a 12V system.
Most modern analog electronics run on 5 volts internally, so some of the newer designs may work ok on 6 volts, it shouldn't hurt them to try. They should have a regulator inside to convert the voltage down, and most off the shelf regulators can work with only 1V of headroom. If I was designing a new tach {I actually design instrumentation for a living, but much more complex stuff than tachometers}, I would design it to have 5 volt supply rails internally and use an LDO (low dropout) regulator to derive the internal voltage from 6-30V dc, so it could be used on 6, 12, and 24V systems.
If you can find the schematic for a tach you are thinking about using I'll be happy to tell you if it can be modified to work on 6V if you can email it to me.
