It'd be pretty simple to accomplish: place fill as your primary, then take the role you'd LEAST like as your "secondary" which would be the exclusion slot.
Mid/Fill, you want mid, but will fill any role.
Fill/Mid, you will fill any role, but you DO NOT want mid.
It's not too hard in theory. And tbh, I'm getting quite annoyed at the argument concerning queue times, because the same people complaining about high queue times are the same ones I see complaining about their "braindead teammates" that more than likely got autofilled into their off-role. It's quite simple really: pick the lesser of two evils (waiting for a game where you know everyone's content with their role) or take a break from the game and come back when you're ready to play the game instead of complain about every aspect of the game... >.>