Here's a fact, I experienced last night when a friend wanted me to help him tradevolve some pokemon (Rhydon, Dusclops, Scyther). I had yet to beat the 7th gym, but he was able to trade them with me, and evolve them effortlessly. We even disconnected accidentally part of the way through, and the Pokemon stayed in my party like normal.

Afterwards, I wanted to see the Pokemon that evolved again, cuz they were pretty trippy (particularly Rhydon's evolution). However, they do not show up in my Sinnoh Dex, and I can't access them at all. I hope they will show up when I get the National Dex.

Regardless, you can get any Pokemon in a traditional trade, regardless of if you have the National Dex. They just won't show up in your Pokedex.