Well, I sacrificed a webcam in the end - prised out the tiny IR filter from the back of the lens (not over the sensor as I'd expected) and stuck two bits of Congo Blue and one bit of Primary Red over the lens - and away we go.
The monitor is displaying an image - but you can't see it. Between my fingers is a bit of film - you can just about make out the shape of it faintly. In visible light that's so dark blue as to be almost opaque.
In this one the telly's on but again you can't see anything. The foliage is reflecting the light from the uplighters rather nicely. The various remote controls light up beautifully and make quite good torches for poking about under the desk.
Wheee!