I'm pretty sure it was mentioned somewhere before so I really don't take credit for this, but I found out how to evolve a Pokemon that only evolves via trades by yourself! What you need:

1x Pokemon Heartgold/Soulsilver/Diamond/Pearl/etc. (Any Pokemon game that can connect to the GTS.)
1x A Nintendo DS
1x Wireless Internet Connection

For this example I'm using the newest games HG and SS. Once you have access to Goldenrod City (In D or P it's Jubilife City.) head to the Global Trading Station (GTS for short.) and talk to the woman at the far counter. It should ask you to save your game before allowing you to access the Nintendo WFC. Go ahead and save your game and log into the Nintendo WFC and it will connect you to the Pokemon GTS. Once here choose to deposit a Pokemon. The Pokemon you want to deposit is one that only evolves via trades. When it asks for a Pokemon you want, put something that's unlikely. I myself put a Gyarados, Lv9 and under. Overall it really doesn't matter, only lessens the chances someone will trade for yours. Soon as you confirm it and send your Pokemon on its merry way immediately log out of the GTS and disconnect from the Nintendo WFC.

Now, immediately talk to the NPC again and reconnect to the Nintendo WFC and log into the GTS. Choose Summary when you connect and it will give you the option to take back your Pokemon. Do so and it will return your Pokemon to you. Now here's the trick, when your Pokemon returns it should say it's evolving. Just let it evolve and you'll have successfully evolved a trade-only Pokemon! I haven't fully tried it but I've found when I tested it with my Graveler then promptly removed it the Pokemon evolved into a Golem, no trading between other players required. The important note is that you MUST disconnect from the GTS and Nintendo WFC, just depositing and retrieving in the same session doesn't seem to work. I plan to go capture another Graveler and test it to be certain but it's worked for me.

EDIT: Unfortunately it didn't work this time, there's more to it but I know I didn't leave my Graveler in there for more than 10 minutes before retrieving it and having the bugger evolve.

EDIT 2: Apparently a trick mentioned by another site I checked suggests you need to deposit your Pokemon then trade a DIFFERENT Pokemon via the GTS with someone else, just look for a common Pokemon or something and trade for it. When you go to retrieve your Pokemon you'll find it has evolved. Currently trying to confirm this.