It has taken quite a few months since
I asked whether anyone had already tried making a Yocto build for the Clara HD but I've finally got it working and in a state that's close enough to respectable to show other people.
The Yocto layer is at
https://github.com/kevinior/meta-kobo-clara-hd It's built on top of NXP's imx-linux Yocto stuff, I'm planning to try to get rid of that and just use a standard Yocto release. That should allow upgrading to a current Yocto version too.
The main purpose for all this was to build my own Home Assistant dashboard running on the Clara, without any leftover Kobo software getting in the way. You can find HAslate here:
https://github.com/kevinior/haslate
Many thanks to
andi45 for his patched kernels with Clara HD support.