Anyone using an IR or Astro modified XE1 or XPro 1 or any Fuji camera?

In addition to time-lapse, I'd love to have arbitrary-length exposures as an option (similar to Phase One's "Xpose+" technology).  The technique is simple: record a full-resolution video for the time the shutter is open, but instead of recording each frame just add it onto a single-frame buffer containing the total brightness per pixel.

It's conceptually the same as taking a series of exposures and then averaging them, but in-camera it would be a great feature since it doesn't waste the processing power and storage of writing each frame out, and it doesn't close and open the shutter between frames leaving gaps.  A side-effect is arbitrary dynamic range; you just need to expose long enough to overcome the photon shot noise in the dark areas.