I figured that it might have been related to the Ubicom processor in the DIR-655. This is one the reasons cited by people in the DD-WRT project as to why their code will never work on the DIR-655.
However, it looks like the DIR-615, which has its firmware source code released, does have a Ubicom chip as well, though probably a less powerful one. I'm just guessing, based on it having a built-in QoS engine, and obviously I may be wrong.
Since the DIR-655 is a really great Gigabit and (2.4GHz) wireless N router on the market, expanding on its functionalities would be a lot of fun. As it is, I can only hope that we still get new and exciting features in continued firmware development.