Redirects act as "alias IDs" to entities. There should be a convenient way to get them from the API (in terms of wikibase entity IDs, not mediawiki page titles). wbgetentities could have an option for including incoming redirects.

Note that the redirect info should probably be outside the entity structure itself, to keep round-trip consistency with wbeditentity, as well as conceptual clarity about the fact that these redirects are not part of the entity as such.