Found this out by accident. Hope it's not a repost or anything. It's actually very simple:
1. Queue up for a random dungeon as a premade;
2. If it's not the random you wanted to do, queue for coren direbrew (brewfest);
3. (Kill coren, not sure if you HAVE to kill him, wouldn't mind if someone tested this for me);
4. Queue up for a random again;
5. ...
6. Profit!
It has to be a premade otherwise people won't join coren direbrew if you just started yer random. It is NOT a requirement.