expandUnoRcUrl() on
"vnd.sun.star.expand:$UNO_USER_PACKAGES_CACHE/registry/com.sun.star.comp.deployment.configuration.PackageRegistryBackend/registered_packages.db"

returns

"/registry/com.sun.star.comp.deployment.configuration.PackageRegistryBackend/registered_packages.db"

does this look correct? (Linux here)