Sadly, that doesn't look like it will work. The '.backup' command needs a filename and won't accept '-' or even '/dev/stdout', at least in the version available on Debian it will simply error out with "missing FILENAME argument on .backup".
As noted '.dump' will work in your pipeline, but with the caveats I and others noted elsewhere.
As noted '.dump' will work in your pipeline, but with the caveats I and others noted elsewhere.