After several tries, I have successfully compiled, installed and booted a kernel. The issue that was preventing it was my /boot partition–not anticipating my involvement in the OPW kernel project, I created it with 256M, which I thought would be enough space for all the user kernels I would ever need. The initial ramdisk that the build process created is 187M, as opposed to the generic ones from Ubuntu which are 30M each. Those “no space left on device” errors weren’t some silly permissions thing after all, they were real! (Why did I create a /boot in the first place? Because I could, I guess.)
So I was up all night downloading, burning, backing up, formatting, installing, restoring, cursing, diagnosing, cursing some more, and finally admiring the beauty of my new flat partitioning scheme. The white hot moment of anxiety was when I booted into the Unity desktop. Ugh! I finally got a nicely usable system going with Mate (using the nouveau video drivers instead of the nVidia proprietary ones–an unconfirmed, but likely, contributor to my problems). Plus, the kernel spirits gave me a nice little reward for my trouble. When I ran lsmod, I found BTRFS loaded! Sweet!