Great discussion and wonderful, detailed, and illuminating article.

Here are three factors that make an English-only rule more possible - at least in an ESL context as opposed to an EFL context.
1. Partner students with speakers on another language. If you have a truly multinational, multi-lingual class, English becomes the best - and only - way for students to speak to their classmates. This global classroom aspect makes teaching English in universities often more satisfying. I currently have students from at least 15 countries.
2. Allow students to bring in both paper and electronic dictionaries. This allows students to find missing words without crossing the room and asking a fellow speaker of Russian, Korean, Spanish, or Chinese.
3. If you can't cajole students into working in international groups with soft words, than you have to assign both seating and groups. Many university students - especially in engineering - are used to working on group projects and plan to work on international teams where English is the default language. Emphasize English as the global tongue - in the real world and in your classroom.

What if everybody speaks another language? I don't know the answer, but I would be tempted to challenge the students to create a solution. State the problem. Elucidate the short term and long term consequences. Ask the class to find at least five solutions, evaluate the advantages and disadvantages of each one, and make a recommendation. I might even have a class election to select our class policy to enforce the English-only rule.
Would it work? I don't know, but this exercise would teach critical thinking skills and democratic values ... and probably reduce the amount of L1 chit-chat too!