There are programs that can convert the Bayer array (pre-demosaicked) data to a grayscale TIF and you could probably manipulate and combine those TIFs in Photoshop. I think at least one of those programs can then do the demosaicking of the combined file, but it's been over a decade since I've used a much earlier version of the software so my recollection may not be 100% and the software may have changed, and nothing that I know of can insert that combined (pre-decmosaicked) data back into a DNG or raw file, so I suppose you can write the software to do that part of the process.
Unless you are a spy agency or criminal enterprise trying to alter photographic evidence where the modification is forensically undetectable, I'm not sure what the purpose of working with pre-demosaicked data is, but I'm sure you have a good reason. It's just that no one else does, so there's no a market for such software. The software that can create a TIF containing pre-demosaicked data from a raw file is RawDigger, and a program that can possibly do the same thing, plus perhaps demosaick the grayscale TIF into a color TIF is ImagesPlus. RawDigger is for raw analysis, and ImagesPlus is for astrophotography.
And the trouble you're having has nothing to do with the DNG spec, the trouble is the data is raw data, rather than RGB data. A DNG can actually hold RGB data like a JPG, as well, but it's not raw data, just RGB data. If you're just trying to trick someone that's not very bright into thinking they are looking at a raw file when it's actually a TIF or JPG then you can open a JPG in the CR plug-in and save it as a DNG, but I don't think that's what you're trying to do, even though it is data in a DNG it wouldn't be raw data, and the ACR plug-in nor Lightroom would treat it as such. Unlike a DNG containing raw data, a DNG containing JPG (or TIF) data would have only relative white-balance adjustments and no camera profiles, just as if you were adjusting the JPG in ACR or LR.