sounds like a hardware function provided by olympus, but wonder if this can be implemented/mimic it with multiple shot and real time stacking preview.
to describe the function, it is simply showing what you get in a bulb exposure, and you can decide when to stop the bulb exposure judging by the preview at the screen.
if in ML, can we set the camera to give preview of the stacked sequence. once you first take a shot and hold the shutter button, it continue to give you a previous, and when the next frame is captured, the preview shows the stacking with the previous frames in the sequence.
in dark environment, you will see the picture getting brighter and brighter if you continue the exposure.