When building a camera, you decide once and then most parameters stay fixed. It would be trivial to just append 1000 bytes for a mostly fixed DNG header to each image.

But how do you test this? While the DNG specification is open source, the implementation was/is(?) not. Do I really need a copy of Photoshop to test if my files are good? How would I find good headers to put into my files? what values are even used in processing?

Maybe the situation has changed, but in the old days when I was building cameras there was only a closed-source Adobe library for working with DNGs. That scared me off.

Things like camera intrinsics and extrinsics are not fixed. 1000 bytes seems small to me given the amount of processing in modern cameras to create a raw image. I could easily imagine storing more information like focus point, other potential focus points with weights as part of the image for easier user on device editing.

For testing, there's the Adobe DNG SDK: https://helpx.adobe.com/camera-raw/digital-negative.html

You'll find the whole spec there, too. I think the source is also available somewhere.

The camera app on my GNU/Linux phone stores DNGs with no troubles using FLOSS only.