Okay, I found this thread: https://groups.google.com/forum/?fro...3d/vfzbQqUeOwQ
So I created a blank white image in PS at the specified resolution, saved it as a 32-bit tiff, since the bit-depth in the Attr Editor was 32. I tried a render and the image now looks like it has some grayscale information for my render. So that's cool. I turned off writeable and am now rendering again. I think it might be going okay, but the output window says this:
PHEN 0.10 193 MB error: misss_lightmap_write: Lightmap 1073741828 is not writable, aborting lightmapping.

Is that expected? I guess I'll just see how the render looks.