Inspired by this question.
Are there any cameras / firmware hacks out there that read the sensor multiple times during a long exposure, and show the partial picture on the screen?
Light-painting should be fun with such a camera :)
Yes, actually. The Olympus OM-D E-M5 has a "live bulb" feature which does exactly this.
This shows previews at intervals from half a second to a minute during a bulb-mode exposure.