Group: Hardware/research/e-readers/Kobo/Clara HD
The Kobo Clara HD has a device tree file (imx6sll-kobo-clarahd.dts) in Linux.
It's however not complete yet as for instance the display is missing (this would require to port the Freescale/NXP driver to the upstream kernel).
There is a blog posts  that shows how to install various free software on top of the stock OS (which contains nonfree software). The person explaining that installed Koreader, so we can assume that Koreader somehow works with the vendor kernel.
There is also a website  that explains how to replace the internal microSD.