I most likely won't be able to work on this for a while, so I'm posting what I've got so far. USB Host mode is working, I tested with a USB keyboard, USB mouse, and USB flash drive. They all work as long as you provide power (for example with a powered USB hub). But the kernel is Host mode only. This means that you cannot manage your Kobo with your computer via USB anymore, since that requires device mode.
This is possible thanks to the great work of some people over on the
Top Hat forums, who based their work on the Kobo XCSoar USB Host kernel. The kernel can be found in the
USB-hot-plug branch of the XCSoar-Kobo-Build GitHub repository. My kernel config file, which has the selections needed for USB HID devices, I've
uploaded on PasteBin.