Editing a DNG file and saving it again as DNG using Photoshop

Question

Hi everyone,
I have a set of .dng raw images which I would like to edit using Photoshop and save back out as .dng files.  It sounds like a very simple task, but I have been struggling with this for half a day.
To give a concrete example, I have two dng images from the same camera. I want to copy an object from one image and paste it to the other, and then save out the output as a .dng file.  This should take me like 2 minutes in Photoshop, right?
Well, when I open the files, it first brings up the Adobe Camera Raw tool, but this only lets me do very basic edits (I can't figure out how to copy an object from one image to another, let alone perform the necessary edits to blend them seamlessly.).
So I click "Open Image" to get them into Photoshop, where I can do all of my necessary edits (copy and paste, brushes, etc.).  However, after I am finished and I try to save my edits, Photoshop only lets me save the output in .psd (along with other formats) but not dng.  I need to save it back out as .dng, as the next program in our workflow pipeline requires .dng files as input.  So how do I do this simple task?
Why can't I save my resulting image back out as a .dng file?  I tried saving as .psd and then using Adobe DNG converter to turn it into a dng, but it doesn't seem to be able to do that.  I also tried saving as Photoshop RAW, but for some reason the converter can't convert from Photoshop RAW to DNG.  It doesn't make any sense to me.
I know that Photoshop knows my camera parameters because it was able to read the original dng files properly in the first place.  So why can't I read in the .dng image make some edits and then save it?  How are people editing dng files?
Thanks for your help.

Answer 1

Also, there is no way to turn an image back into raw image data after it has been edited in Photoshop. It would be as difficult as uncooking an egg.
I understand that this is difficult if I am doing arbitrary image editing operations, since those operations may not be properly defined in the RAW space.  But in this case I am cloning pixels from one .dng image to another.  Shouldn't this be possible?
For example, assume we have 2 dng images and we want to take the left half of one and paste it in the same position over the other.  I am not familiar with the dng spec, but at some level it should encode the values at each pixel (perhaps with the appropriate camera curve).  So I just simply want to overwrite the values of one .dng image with values at the same pixels of the other, and since it's the same camera with the same settings used for both this should be possible, right?  In the worst case, I could write a program based on the DNG spec that parses the two dng files, overwrites the necessary pixel information simply using the second image's data, and then writes the dng back out.  But this seems like a complicated way to do something that there should be an Adobe tool to do.
Perhaps it would help if I tell you exactly why I need this for.  We are capturing raw video using the Blackmagic Cinema Camera which outputs a directory full of 2.5k dng images for each take.  We were doing a take of a scene using a tripod, but an otherwise great take was ruined because at one point a crew member poked their head into frame.   So I want to use information from the other dng frames to overwrite this portion of this image to hide this problem.  
If these were regular images (.jpg, .tif, .exr) it would take us no time in Photoshop to use the information from the other "good" frames to fix this problem seamlessly.  However, we would like to keep everything in dng format so that I can then read everything into DaVinci Resolve for color grading correctly.   Any idea on how I can do this, if I can't use Photoshop?
Thank you in advance for your help!