MusicBrainz could itself function as a kind of “Rosetta Stone”, I guess? We have ISRCs, ISNIs, IPIs, ISWCs, bar codes, and label codes directly attached to their relevant entities. In addition to that, we have URL relationships which allow linking to Discogs, AllMusic, Spotify (not sure if it’s the actual Spotify ID in the Spotify URLs though?), VIAF, WorldCat (OCLC IDs), Wikidata, iTunes, and a number of other external services which has the ID in their URL.
Of course, this is far mostly manual input - though some of it can be semi-automated with e.g. user scripts running in the browser or automated by setting up a bot.
Edit: Also note that we have access to a data dump from ISNI with their ISNI→MBID mappings, which could be fed to a bot and automatically fed to MusicBrainz if someone were to write such a bot; see Notes from #MetaBrainz Meeting on 2016-04-18