Is your feature request related to a problem? Please describe.
I have a uint16 image data from a thermal camera and I would like to save a paletted jxl file where I specify the palette. The palette would map the 16 bit values to 2^16 RGB colors of my choosing. This would be ideal as it would allow me to glance at the image files with a standard image viewer that would support jxl files and understand the image, while preserving the raw thermal data for scientific usage. I noticed there appears to be a reserved channel for thermal? I am unsure of how to use it with libjxl

Describe the solution you'd like
To pass my uint16 PNG and an associated color map to generate a paletted jxl image.

Describe alternatives you've considered
I have looked at several file formats and this one appears to be the most promising due to the compression and support for large palettes.

Additional context
Add any other context or screenshots about the feature request here.