This probably needs to be a magic comment, something like:
# Provide default completions
The other option is to have sub <command> --completions echo out something like [files] or [directories] etc as required.
sub <command> --completions
Yeah, I'm actually wondering if this is an issue. You can always listen for the --completions flag, and exec ls or run a quick find. Going to close for now.
IMO zsh's completion is far better than anything we can come up with using exec ls, however as I haven't the time to make a patch, I'll let this one go for the moment. Maybe if I get a moment I'll sort it out.