Yeah, this, basically.
The OP specifically states that a player should be able to create a character sheet for any race on the race sheet, so long as they get it approved by the race's ambassador. And making sure that a race is something that other players can create characters for is a pretty big thing that we keep in mind when reviewing race sheets. o_o
So, yeah, I guess you can technically flesh out the lore of an extinct race, if you want, but it won't count as a playable race.
And, as Luma said, I recommend you just include the info about the extinct race in a spoiler tag on the existing race sheet -- since I'm assuming it's meant as additional lore to go along with your other race -- and, if it's in a post by itself and never added to the race list, then it will soon be buried and hardly anyone will know about it...