Lossless JPEG XL is in the DNG spec since three years (v1.7). So, you can convert your proprietary camera raw files to smaller ISO standard compliant JPEG XL compressed DNG raw files (mosaiced, not linearized).
Unfortunately, at least as of perhaps a year ago, I couldn't find any other actively maintained software tooling that could reliably do what Adobe's DNG Converter was doing, and there's so much nuance to the world of RAW files that's not at all just a trivial one-shot prompt to Claude/Codex to roll your own.
Not linearized as in samples do not represent linear color space? I think that's untrue, as all sensor work I've done (a decent amount across many projects) are linear: double light = double photon count = 1 bit of sensor space. There are technically often some bends from physical and engineering effects (photon interaction with substrate has some variability, temperature affects things a bit).
Or is there some other meaning of "not linear" you mean?