In case anyone else has trouble with this: I had formatted my NVMe *device* in Genode, which created an ext2 filesystem on the raw block device. This is technically valid, but I mistakenly assumed it had created a single partition which is more common. I was getting errors about partition 2, so I changed the label to "1" to indicate partition 1, but that still didn't work.
After booting from a Linux ISO, creating a GPT partition table, and creating a single ext2 partition, I was able to go back into Genode and make it work.
Not such a difficult problem to track down in the end, and it's been helpful exposure to how the XML configuration works.