Thanks 419 -- I was wondering how the cameras could get an image without a flash -- IR will do the job, though I doubt if there's such as a thing as IR flash.
We can look forward to some IRCM such as Heated or Cooled Plate Covers so that plate shows as white or black spot -- unless plod has IR vision, won't be able to see
You need a thermoconductive visible light transparent substance to blur the IR image -- such as
water.
A more concealed option would be a heating element behind the numbers/backing, perhaps with a mask to create a differential temperature to compensate for the different radiativity of the backing and numbers.