Yesterday I was doing DTK for the 4th time that day (Yes, I was very bored) and was in skeleton phase as normal. I was on my death knight and KM kept proccing. I accidentally hit my Howling Blast key as it's bound near the spells you use while a skeleton and it cast it.
I'm going to take a crack and say this isn't working as intended since your bars get changed when you turn into a skeleton.
Later when I get another DTK group I'll take a screenshot / fraps to prove it, but for now, try it yourself I suppose.
IN SHORT: Spells (abilities too?) work while you're a skeleton on the last boss of DTK, Tharon'ja. Just make sure they're not on your main bar.
Picture for proof.
http://img205.imageshack.us/img205/4345/hwskelezs5.jpg
by wolfiewolf666