I don’t know that it would be possible with the current player base. There just isn’t enough people still playing the game.
And just as a comment on this, Blizzard discussed this in a Blizzcon interview (I don’t remember when, or where, so don’t ask for a link) and said that one of the biggest issues with adding a “Solo only button” is that the way queues work in hots, it wouldn’t be able to put “solo players who don’t select solo only” into both the “solo only queue” and the “play with teams” queue, it would have to put them in one queue or the other. And since they are allowing 4 man groups, they would have to put them in the “play with groups” queue, meaning that the solo play queue would ONLY contain players who checked the “solo players only” box, and not players who play solo with the box unchecked (if that makes sense.)
So the end result would be MUCH higher queue times, as you would basically be going back to having a HL and a TL, just with a shared ranking.