I know you said this is more for season 4, but my theory on when Dean became the righteous man:
Dean became the righteous man when he broke on the rack. John also could've been Michael's vessel, since it's passed through the bloodline. John is the eldest (also only, that we know of) son in his family. Maybe that's why they tried to break John? Or if they broke John, maybe he would've tortured Dean on the rack in addition to Alistair.
When Dean broke he dinged on Heaven's radar as "the one"
Sorry for getting ahead, but this week's episode just brought up so many thoughts about season 4.
I like this theory we discuss it a little more in our 4x01 episode too I believe and John and righteousness.
Woah Anon tho.....I love that you made this worse XD
"Or if they broke John, maybe he would've tortured Dean on the rack in addition to Alistair."
I've never considered this could've happened and ouchies I'm glad John crawled his way out of Hell if it meant we avoided this!
I think the idea of Dean breaking being the point at which he becomes righteous is really interesting, I would love to hear more from people on this.
My personal take is that the sacrifice for Sam was the point at which he becomes 'the righteous man' then Heaven was just playing a waiting game to see if he would break the seal, maybe that's also why John could be also - he technically made the same sacrifice for Dean?
I wish they'd explained it a little more - I like my lore to be concrete!
But then I guess we wouldn't get to have cool discussions like this one :)
Also, I am digging the enthusiasm for S4, I hope we don't disappoint!