I am by no means an expert in sensor technology.
Keep this in mind however: Sensors unlike film are designed not necessarily to react to light per sei, but rather to record the intensity of light. If you take a picture of the sun, damage WILL defiantly result unless you use a Sun Filter which they made many a moon ago. This damage is caused by the intensity of the light, as well as the magnification effect from the lens. (Think magnifying glass and ants.) So I am assuming you would know this.
Thus the longer the exposure in lower levels of light, the ‘cooler’ the sensor actually will be. (Heat also dissipates.)
Unlike film that will ‘burn up’ with light exposure over a longer period of time, the sensor works off of the electronic signal coming from the ‘pixels’ themselves. All that will happen is the longer the exposure, the more light is recorded in one spot. Thus, the image becomes lighter and lighter until, pure white is achieved. The image recorded is in the form of ones and zeros. Thus, long exposures wont damage the sensor in that fashion.
HOWEVER!!! The sensor is an electronic device, and that by default results in electrical current, thus heat is the result. If such heat has no means of dissipation, then the material of the sensor could theoretically become damaged. I do not know what the material is in most of the sensors, but logic dictates, (and I am treading on dangerous ground here) that the engineers knew this and thus developed the sensor to deal with such heat build up.
The simplest example I can give is the KM 7D that will shut down if the sensor moves around too much in its anti-shake feature. That action generates a great deal of heat. (I have felt the camera when this happens, and it can get quite warm). KM engineered the camera to shut down in case of such a situation. It does work.
