So, thanks to a bug blizzard introduced in 4.3 and trasmogging, you can unbind any item. I've tried this 3 times so far with a 100% success rate.
Note: This DOES require a GM, yes. However, it's never failed and unless you make it blatantly obvious it will work without any trouble whatsoever. I realize that GM exploits are not allowed here but this one should take special consideration because of how simple and efficient it is. So before reporting it maybe you should read through it.
As of 4.3, random items are becoming soulbound in people's bags regardless of whether or not they have been used to transmog, wear, etc. Blizzard stated this is a known bug and that if this happens you should put in a ticket about the item.
Because the GMs seem to never check how it became soulbound (since the soulbinding bug seems to not generate any event logs on their side), they will unbind any item for you.
1. Have the soulbound item in your bags.
2. Put in a ticket stating something along the lines of;
"Upon logging on, my [xyz] has become soulbound. I have read on the forums that as of 4.3 random items have been binding to players upon logging in and I believe it has happened to me. Could you help me out by unbinding this item?"
3. Wait, and your item will be in your mailbox, unbound!