Mike,

  Don't know much about the operation of the pins on the ECU and all that,
but mechanically, I have some 'guesses' about why you see "rich" on your
Exhaust Gas meter upon protracted deceleration..  One cause could be the O2
sensor is not reading "Fuel-rich" from the exhaust, but rather is seeing
some "blow-by  Rich" coming into the exhaust stream.  The O2 sensor is not a
very sophisticated piece and can not tell the difference between Oil/air or
Fuel/air.
  I'm not familiar with the emission control system on your WBX motor, but
most vehicles dump the blowby into the intake in order to burn it rather
than letting it go to the atmosphere.  If perhaps that is going through your
engine, unburned, and being read by the O2 mixture meter you have, that
would explain the rich indication.on protracted downgrades.
  My 928 racecar motor had a tendency to whip up an oil mist in the
crankcase at very high sustained rpms.  As configured from the factory,  the
camshaft covers were vented back into the intake to burn any excess oil/air
mixture that found it's way past the lifters and valve guides.  We
discovered, using an extra sensitive Exhaust Gas meter on a dyno, that on
'drop throttle', my own system O2 sensor was seeing  "Rich-Mixture" because
of the excess hydrocarbons from the oil mist coming into the intake and
blowing out the pipe...then the ECU would tell the motor...."GO Lean" which
was not good, at high rpm..We routed the breather system into a catch can to
avoid 'confusing' the O2 sensor...
  I had one of those cool little sequential-dash readout  mixture meters,
too.  Fun to see the little lights going up and down..and it was of some use
to tell me when the motor had reached operating temp. and the ECU had
stopped 'searching' for the Stoichmetric perfect mix.
  That car had a separate 'drop throttle' switch..Which, of course, we took
out to race.
   Have you considered that your meter might just be cooling down some and
giving you a different value after you've coasted a little?   It's my
understanding that the sensor's range is very small, very....sensitive.. for
want of a better term..
  Don Hanson