<<Has anyone tried mechanically linking the throttle to a variable
  wastegate on the manifold between supercharger and carburetor?
     I was thinking that this way possibly the more-opened-up wastegate
could
  let the pressure blow off when the throttle was pulled back, and increase
the blow-off level as the throttle was opened.
  Terrence>>

It might be count-intuitive, but the power required to run a supercharger
goes up if the flow rate goes up such as from dumping air downstream of the
supercharger.  Throttling the air upstream of the supercharger will reduce
power requirement effectively, but I'm not sure if the shaft seal in the
supercharger will work with a partial vacuum in the unit.  Some people have
played with pre-swirl as a way of reducing the pressure ratio and that can
be effective, but not over a wide range.  The basic problem is that at a
given rpm a dynamic compressor creates a more-or-less constant pressure
ratio.  An aircraft wants to run a variable compressor speed as the pressure
ratio changes a lot from sea level to altitude (you probably want to run a
constant crank rpm and an constant manifold pressure).  A turbo handles this
task relatively well, but it's not perfect either.

Gary Casey