Snappy/Bigdaddy/Alan need default apps installed. This—it turns out—is a common requirement. We'll be able to work-around this for now for Alan/Bigdaddy but it would be nice to have an elegant solution going forward. This file private-overlays/overlay-snappy-private/chromeos-base/chromeos-board-default-apps-snappy/chromeos-board-default-apps-snappy-1.0.0-r1.ebuild does this:

src_install() {
  # Install json.
  insinto /usr/share/google-chrome
  doins "${FILESDIR}"/default_app_order.json
  doins -r "${S}/json/extensions"

  # Install extensions.
  insinto /mnt/stateful_partition/unencrypted/import_extensions
  doins -r "${S}/crx/extensions"

  # Install validation files.
  insinto /usr/share/import_extensions
  doins -r "${S}/validation"
}

This bug tracks support in cros_config to allow model-specific forking of the json file to install.