Never done it, but it's good enough to cook a cake in an Easy Bake Oven...
I'm not sure if I'm on the right path for the conversion but 1 watthour is equal to about 3.4 BTUs.
Figure a 100 Watt bulb at 5% efficiency that means the bulb is tossing 95W away as heat.
Over an hour's time - 95 watthours
that would equal to 323 BTUs.
In terms of radiant heat, which is cumulative to the point of the maximum temperature being radiated, that could get pretty warm over a period of time.
I'd hook up a thermostat just so I'd know how hot it is getting in there - even if it never got too hot, I'd still want to know.