Hi corre,
Thanks for using our plugins. I can try to help you out here, but unfortunately, you are asking for something that is beyond the features we have. The Events Calendar on each multisite is a separate calendar and we do have any way to share events, venues, or organizers. So in answering your question:
What is the best approach to achieve this without duplicated content (auto creating redundant venue event posts through code)?
If you do not want to duplicate content I am afraid we do not have an option for you in our plugin and the filter bar would not give you this feature either.
If you would like to request a shared calendar on multistie as a feature please head to our UserVoice Page and add your own or upvote that feature as they will increase the chances of it being added to a future version.
http://tribe.uservoice.com/forums/195723-feature-ideas
Let me know if you have any follow up questions.
Thanks