I found an error with my code; I can now read the data. Thank you for your prompt reply.

However, it appears to be Bayer sensor mosaic data; does libraw do conversion to adjusted, de-mosaiced 16-bit image data? Or is that something I am expected to do from the bayer mosaic data?

Here's part of the image as displayed in Adobe Lightroom:

http://fyngyrz.com/images/from-lightroom.png

Here's what I'm reading from more or less the same region:

http://fyngyrz.com/images/from-libraw.png

I have not done a black level subtraction, inasmuch as that would dim the image even more.