Here's my solution:

If someone queues for an in-progress party, then drops out instantly. Remove them from in-progress queues for the next 3 hours (or some tentative time frame, whichever seems fairer).

It's obvious if they drop an in-progress queue the first time, it is highly unlikely that they are looking to help people stuck in a dungeon, they are doing it for their own benefit. Then place them in brand new queues, revoking their right to join in-progress for the tentative time frame stated.

I don't have such a problem myself, because I pretty much forget the Join In-Progress option exists, and only use it if an FC mate or a friend specifically requests me to join their in-progress party.

While this might cause more parties to eventually abandon duty, but you really can't solve all the world's problems. You simply have to pick the bigger problem, and pacify that first. A way to probably help with the abandonment rate of in-progress parties, would be to hide the number of bosses downed for that dungeon. If someone then gets in to an in-progress party, and drops the dungeon, ban them from queueing altogether for another tentative time frame.

Basically, if you are queueing for an in-progress party, you do it to help people. If you are not sincere about helping people, you get locked out, and get clumped to the fresh dungeons queue.