Here are the cases where close() will generate a ZIP64 archive and an exception will never be raised:

1. There are more than 65535 files in the archive.

2. The start of the central directory is at > 2 GB.

3. The central directory size is > 2 GB.

#1 could be checked from write/writestr.  #2 could possibly be checked from write/writestr by looking at the file offset after writing the compressed data.  #3 cannot be checked until close, but I'm not sure if it can ever occur without triggering one of the other checks first.