Using the USD Render ROP and its Husk Post Frame script, is it possible to access pixel data from the frame buffer before it gets saved to a file? I'd like to write some info about it into the exr metadata.

No, I don't believe so, but you can read in the image from the exr file on disk and I think there are command line tools to add/edit metadata in an exr file without altering the image? I'm afraid I don't know what these commands are, but I bet someone else around here does...