Wikidata can be used to obtain a map between Twitter and Mastodon ids for notable people and organizations: http://tinyurl.com/ydg6hz8t. The mapping itself is quite small, but it covers highly-followed users. So I think it could potentially be a nice addition to the OAuth-based approach.

If this mapping is integrated, it would need to be refreshed periodically. Also, this mapping is of course not authenticated by the account owners themselves, so there could be some impersonation issues.