I still don't know anything about info format, but I just looked at
/usr/share/info/cppinternals.info.gz, which has, on line 5:
* cppinternals-4.0: (cppinternals-4.0). Cpplib internals.
sed -e '5s/^/START-INFO-DIR-ENTRY\n* cppinternals-4.0: (cppinternals-4.0). Cpplib internals.\nEND-INFO-DIR-ENTRY\n/'
Maybe this isn't the right solution .. but hopefully a start.
Have you looked at the package: linuxdoc-tools-info?