When I got a D800 around launch time, even Lightroom didn't have the capability to import them directly at launch and I had to convert them to DNGs first. What a bloddy mess.
But once you have that file format, camera manufacturers tend to embed hunks of data in the raw file about settings and other things that the camera knows about the image, like the focus points, white balance, camera calibration, image compression, dot layout and all that.
I'd like to leave that as Someone Else's Problem. (unless there's a lot of money and shiny cameras in it for me)
Freeimage looks like more than we'd need, and includes libraw. And actually, looks like it does a lot of the same things that Pillow does, wrt to various image formats.
Surprised you weren't aware of libraw (assuming DNG/RAW has been a requested feature for Pillow) -- but great it's on your radar now. FWIW libraw does an excellent job of handling RAW/DNGs -- as far as I know it's still the best tool for that (including various closed tools).
I'd known about dcraw from a ways back, just not the libraw fork.