I could see this sort of technique as being useful for weddings:

HDR Video on Canon 5D Mark III - "Bodie" - YouTube

The only drawback is that HDR video with Magic Lantern on the 5D3 that the effective frame rate is only 12.5fps. It alternates a frame exposed at ISO100 with one exposed at ISO1600 & then the post processing combines & interpolates frames to bring it back to 25fps (for us in PAL-land). The technique can be fabulous for static scenes like real estate videos but not so great if there is a lot of movement e.g. bouquet toss. Here is a great example of real estate HDR video this time on the 5D2 Canon 5D Mark II HDR Video on Vimeo

One quick thought: can the HDR be used with 720p50? If so, this might help with (without solving) some problems (motion blur from low frame rates; ease of blending frames), as well as introduce other problems (blending 720p footage with 1080p footage; light loss from higher shutter speed to match frame rate).

re 720@50/60p all of the above plus the fact that the resolution of 720p on the 5D3 isn't great. It could still work for web delivery but would not be so good alongside Full HD footage.

If you really want to do something like this why not use a beam splitter and 2 cameras that are frame locked with different exposure settings?

That is pretty sick...

On a semi-related note. How is it GoPro can figure out HD 60fps in a $400 camera but Canon can't?