Queue specifically for a dungeon where you need an item from the first boss. Kill first boss and leave and you can re-queue because you're not locked.
IMPORTANT:
If you loot/roll from the boss you will be locked
If you queue again and miss pressing the enter dungeon button you will be locked.
Use this to get that pre-raid BiS item you've been trying to get.
Or farm justice points if queue times aren't that bad and there isn't much trash to the boss.
Enjoy!