The solution to this problem is not to to add an "In Progress Only" button. The solution is to make in-progress runs the priority when the "Join In Progress" checkbox is selected. The way it SHOULD work:
1. Player selects "Join In Progress" checkbox and signs up.
2. System checks for in-progress runs that require the player's current class. If it finds some, the player is added to the in-progress run that has waited for a member the longest.
3. If there are no in-progress runs that require the player's current class, it checks for new runs that require the player's current class. If it finds some, the player is added to the new run that has waited for a member the longest.
4. If there are no eligable runs, the player is added to the wait queue.
It is frankly astonishing to me that SE did not code it this way; for in-progress runs, the dungeon clock is ticking and empty slots should be filled as a top priority, well beyond new runs. Nevertheless, this does not warrant the addition of an "In Progress Only" button, it warrants a correction on how duty finder matching works.
My presumption: You get two penalty-free withdrawls per day. Beyond that, each time you withdraw incurs a 30-minute penalty until the 24-hour lockout expires, at which point you get two more freebie withdrawls to spend.