Is it permissible to ask Allah to allow a specific Companion to intercede for me by name, or is it not permissible to designate anyone for intercession except those whom Allah has designated as intercessors?

There is no harm in seeking the intercession of a specific individual among the believers, and this does not constitute an transgression in supplication. However, it is better to seek the intercession of believers in general, to be certain of it. The proof for this is Allah Almighty’s saying: "The angels interceded, and the prophets interceded, and the believers interceded," and His (peace be upon him) saying: "Those who curse much will not be intercessors or witnesses on the Day of Resurrection."