A dark frame is usually taken by placing the lens cap on the camera and opening the shutter for a period of time to determine the noise component of the signal for a certain exposure duration.
I know the camera software is written such that no photos can be taken with the shutter closed, but is it possible, in theory, to activate the sensor with the shutter closed and use the result as the dark frame?
Admittedly this would require a script or software package to be installed on the camera, but is this a viable method for taking a dark frame?