After a month of back and forth with Barnes and Noble support and numerous references to the GPL, I think I've managed to acquire a copy of the kernel source for this tablet. It's published as a ZIP on Barnes and Noble's website. I haven't spend a whole lot of time with it, as I currently do not have the resources to build it, but from a cursory glance it seems to be reasonable, if in a weird format. I've also mirrored the source on GitLab (unfortunately, GitHub doesn't work with files over 100 MB nicely) for you guys to play with in case the Barnes and Noble link goes down or you'd like a faster download. If you see anything crucial missing, let me know and I'll try to get it fixed by pestering them to give it to me!