Since version 4.8 External Keyboard Helper supports Korean/Hangul out-of-the-box. The app has also been fully translated to Korean language by the very helpful person who goes under the pseudonym Neoely. Neoely has also assisted with testing and given me excellent feedback and explanations on how a Hangul keyboard works.
Update 2012-09-23: Here's a blog post from Neoely in Korean language that describes how to setup and use External Keyboard Helper.
The Hangul text on the right was written by me and I used Google Translate to translate the string "External Keyboard Helper" and then use my bluetooth keyboard to type it. If it says something else, then blame Google Translate. ;)