OK, to actually answer the OP:
Best bet is to treat it like you would an ordinary pitch: for best viability of the dry yeast, go ahead and rehydrate prior to pitching into your starter wort. And, maybe to make up for the additional volume, I might reduce the water you disolve the DME in bye the volume you rehydrate your yeast in... (Just a thought there, not strictly necessary, but it could help avoid a messy overflow!)
Now, for my turn at a little additional advice: before going to the added effort of a starter with dry yeast, make sure you actually need that starter. Typically, they're not needed because a dry yeast packet contains significantly more yeast cells than a liquid yeast vial/smack pack, so you've usually got enough cells to ferment without the need of a starter. Check out the yeast pitching calculator on mrmalty.com if you haven't already, and it'll tell you exactly how many dry yeast packets you need - if the number is more than 1, then go ahead with your starter plans. If it's not, I'd say hold off until another brew with a higher OG. Starters are useful tools, but especially with the busy schedule you described, I wouldn't "waste" the time with one if you don't need it.