..Has anyone considered supercharging for the Vanagon?

Yes. I have seen many postings on exhaust-driven superchargers (turbos).

>   It's also my understanding that the boost from a supercharger can be
> 'dialed' to the desired level, right from a dash knob...So if you are
> just donking down a flat road in no particular hurry, you can use Zero
> boost but if you need some power, you just dial it up a bit and cram
> more air/fuel mixture into the cylinders...

Don't think so. The speed of a belt-driven supercharger (and hence the
boost) is set by the diameters of the drive & driven pulleys.