You could have the 2nd, 3rd, and 4 place runner ups of the Papal election gain a claim against the Papacy. You can then try to get one of them to come to your court if he isn't already and set him up as the anti-pope. This way it still makes investing into Papacy race worthwhile even if your chosen Pope doesn't get elected.
When your anti-pope dies another person in your realm with a claim on the Papacy takes over. If there is none, the Anti-Pope title dies out since their are no other people to legitimately claim they should be Pope in your realm.
The thing is, with the mechanics of the Catholic Church extremely simplified, even after the DLC, there's just no way to make that work right. You'd have to invest in a lot of cardinals, hope that they vote for someone in your realm, then press that person's claim. It would make the antipope mechanic nearly nonexistent. I do support the antipope requiring the possession of a cardinal to be created, but any more would be overly difficult for players and impossible for the AI.
And yeah, antipopes shouldn't succeed each other. Except for the Great Western Schism, successive antipopes came from a single ruler with an ongoing dispute against the pope. Henry IV, Henry V, and Frederick I all kept choosing their own pope, but in several cases they waited months between them, just in case the absence of an antipope would bring the pope to the bargaining table. Automatically elected antipopes, as if a papacy-in-exile, would make a mechanic out of the exception and not the rule.