dictionary_generator is the tool to make custom dictionaries, it's built from dictionary_generator.cc in the brotli/research folder https://github.com/google/brotli/blob/master/research/dictionary_generator.cc
dictionary_generator doesn't have any documentation and its presence in the "research" directory indicates to me that it isn't intended for regular users. Therefore, I'm not really comfortable with build and installing it as part of this ebuild. If you disagree, please let me know - I'm happy to further discuss if there's good reason to include it.
I followed this discussion https://github.com/google/brotli/issues/697