So, related:

• The ticket adding external ids to the data model - T133381: Add support for foreign entities to EntityId
• The 2 PRs on github adding the parsing ability for external IDS https://github.com/wmde/WikibaseDataModelServices/pull/146 https://github.com/wmde/WikibaseDataModelServices/pull/149

So, the current docs & blockers that need to be overcome can be seen in https://github.com/wikimedia/mediawiki-extensions-Wikibase/blob/master/docs/federation.wiki

Currently a shared DB is needed / DB access is needed.

Also mixing local and foreign entities currently will not work

Note that as of March 2017 it is only possible for Wikibase to use entities from a single repository, either local or foreign. For instance, it is not possible to use both local and foreign items in statements.

So during the Wikibase-UserGroup wrap up of day 1 we decided to prompt & start a discussion with people that want to use this and try to resolve the current caching & purging issues with labels etc of entities from remote repos.