Hustle
Here's how I got there (story time yay!):
I disagree with the NSA folks instantly, because the benefit of two abilities, especially two that serve disparate purposes, is to make the opponent have to figure out what you're doing before formulating a strategy to take you down instead of knowing right away. Having only Shield Dust makes it very easy to create an all-purpose counter, just like giving CAP 8 significantly higher defense over special defense (e.g. Toxic, TWave, Draco Meteor users, Blissey, etc.). Unpredictability is key in a metagame based solely on prediction skills, and giving this pokemon two radically different abilities would achieve that goal. So, anyway, here's where I'm at with this:
From the list I took out those that just didn't seem useful: Damp, Forewarn, Suction Cups, etc. Then I took out the ones that didn't seem to fit with Electric/Dragon: Chlorophyll, Iron Fist, Solar Power, etc. The remaining "usable" abilities fell into five categories: Weather-Related (Air Lock/Cloud Nine), Status-Related (Early Bird, Marvel Scale, Quick Feet, Shield Dust, Static, Synchronize), Berry-Related (Unburden), Offensive (Hustle, Mold Breaker, Skill Link), and Defensive (Aftermath, Rough Skin, Thick Fat, Trace). From there, it was the process of elimination.
For Status-Related, I figured that the only abilities there that would complement Shield Dust would be Marvel Scale or Quick Feet, as they would discourage non-damaging status moves while Shield Dust eliminated hax status. The other three were either too situational or based on luck. When the opponent doesn't know which ability you have, they'll be a lot more cautious, so Thunder Wave abusers would be very wary to throw it around like they do now. I ended up with Marvel Scale > Quick Feet because I personally love the idea of a dragon being a wall and it worked with Shield Dust more.
For Offensive abilities, I threw out Skill Link because there is no way this thing will run an attack like that. Hustle > Mold Breaker because, as this CAP's SpA will most likely be higher than its Atk, Hustle will give it the option to be a great mixed offensive force (and it's almost certainly getting Volt Tackle, which should be used). Mold Breaker would pretty much only be useful against Electivire, as this guy won't be running EQ for Levitators or anything.
Defensive abilities were a lot harder. I started with Aftermath > Rough Skin as a solution to sub-berry sweepers and weakened LOers. Thick Fat would be epic late-game if this guy gets Magnet Rise and the opponent's dragon is gone, and Trace is pure fun and offers amazing versatility. Seeing how many people use Porygon2 over PorygonZ, Trace is definitely useful. So, in the end, I chose it for the fun factor, as well as it's ability to sometimes play an offensive role.
So then I was at Air Lock/Cloud Nine, Marvel Scale, Unburden, Hustle, and Trace. I threw out the weather ability because it's boring and I could only see it being useful in rain (w/ Thunder). Unburden (as a free Agility) is great for sub-sweeping, which this pokemon is capable of with its great STABs, but if people really want to go offensive with this CAP, there will be other ways to do it, so Unburden's out. Plus, Hustle > Unburden on this kind of pokemon for its unpredictability.
Then I was down to Marvel Scale, Hustle, and Trace. I think CAP 8 should have a high level of unpredictabilty, so Marvel Scale was instantly intriguing. The level of mind games to be played with the opponent with both Shield Dust and Marvel Scale as options is astronomical. Hustle is perfect for mixed attacking and opens this CAP up to both Draco Meteor and (DD and/or) Outrage. Trace is just super fun and can be brought in on a Levitator to nullify its most common weakness.
In the end, it was all about Hustle:
1) It does not overshadow Shield Dust.
2) CAP 8 is somewhat defensive, which means it should be capable of good offense.
3) I am 100% certain Atk will be low compared to SpA on the final spread and Hustle will make physical sets less gimmicky and more of a viable threat.
4) It opens the door for mixed attacking.
5) With the right stats and EV investments, it could make this pokemon one of the most versatile in existence, capable of going hardcore offense or defense, or hardcore physical or special. This also would make it completely unpredictable.
6) It makes Blissey think twice before coming in to wall outright.