SKIDVID6 - COMBINING MULTIPLE EXPOSURES FOR HDR WITH DSLR VIDEO from Robin Schmidt on Vimeo.
Soviet Montage posted some nice demos earlier without details, except that the image was fed via a beam splitter. Richard van den Boogaard says that Soviet Montage worked with E3D Creative, who have rigs for Motion HDR (their demos used AE CS4 for "some tricky luma key"); see Exploring HDR Video – The Real Future of 3D Video.
HDR Video Demonstration Using Two Canon 5D mark II's from Soviet Montage on Vimeo.
Update: FilmmakerIQ notes a video in which "MiLapse shows you how to setup a HDR timelapse shot using the Dynamic Perception Stage Zero Dolly, MX2 DollyShield and Pentax K7."
How to shoot HDR Timelapse from Dynamic Perception on Vimeo.