Special:EntityPage should take an entity ID as a subpage parameter, e.g. Special:EntityPage/Q12345. Special:EntityPage will generate an HTTP redirect (status 303?) to the respective entity's page on that repo.
As a bonus, Special:EntityPage should support foreign entity ids as well. Special:EntityPage/foo:Q12345 would effectively redirect to foo:Special:EntityPage/Q12345 which would in turn redirect to Q12345 on the foo repo.
Rationale: client wikis (including federated repos) should have a way to link to an entity page without the need to know the namespace names of all entity types on the target wiki.