Nick, you could use the old plugs, try clean them up well and at least know what the base ring looks like before you put them in.
IMO heating up the plug and heating up the bike are 2 diffrent issues. You should heat up the engine with a different set of plugs , as a lot of seperated 2-stroke oil that has collected on the crank will be thrown up and sucked into the cylinders, this will definately spoil your reading.
As for heating up the plug, this should definately also be done, preferably with a bit of hard riding before you do the WOT test. The black ring you see at the base of the plug is caused by soot depositing at the COLDEST part of the plug, it is obvious that the base of the plug is the coolest as it is the closest to the cooled head and has the massive heat sink of all the metal around it. Once you have covered all of the ceramic with soot in the case of an old plug it will be more difficult to see the soot ring at the base of the plug.
As for not seeing the ring at the base of the new plug, my guess is that your WOT test was too short.
Where I live I can manage to hold her WOT for at least a minute and I have always been able to see the soot ring on a new plug after the test, provided you're in the ballpark with the main jet allready.