I just finished some testing using qemu-img to convert the qcow2 file to a raw file, just to see if that would work to boot from, which it did actually. So I'm guessing that something goes wrong when the conversion goes straight to the LV. I just wish I knew what exactly...
edit: Just for fun, I tried to use dd to copy the raw file to the LV. Lo and behold, the damn thing could now boot off the LV. A little hassle with this workaround, sure, but it seems to work.
I'm not quite ready to call this one solved yet, but it's a very good start!