Announcing a new game at Gamescom is probably already hard to do with all the big boys around and announcing their new projects ( THQ recently: http://3ds.4gamers.be/nieuws/6306/1/Grote-THQ-aankondiging-op-Gamescom
) but if you want to be in the picture then you must convince the gaming press so they are willing to hear the Project D announcement.
Here some ideas. Maybe to expensive, but it might trigger something else. ^^
Someone already said something about chocolate/pralines. That's a good idea imo. Larian is the only 'big' game studio in Belgium (next to the multiple smaller devs but they do mainly iphone games / small jobs for EA etc.).
You can do something towards the gaming sites that release previews, news, etc. Make a small box (size of a bongo box
or something) put in something that's known from Belgium like a few chocolates. (Many chocolatiers can personalize a praline with a taste, color, etc.).
Also put in a small but fancy voucher that invites the press from those websites to come and hear about the announcement. No long textes or such. Keep it simple. Like "Hunger for more of these delicious chocolates? come visit us at the business area at Gamescom" for example. (just saying something ^^)
As your booth is located in the business area you probably will not reach the gaming public. Find some people who are willing to do:
A) A flash mob. It's popular and everybody gets happy from it. Afterwards give flyers to the public with an website linked to Project D. Do something like Arenanet with their GW2 announcements. Keep a blog or such. No long textes, make small movies.
B) If Project D involves dragons, find a supplier who sells those carnaval chinese dragon thingies. Find some people to run with it around Gamescom. (Maybe a personalized dragon?)
C) I don't know how the Larian Booth looks like. So if it's all fancy and tight why not decorate it with stuff you can hire for a weekend? Things that are found in the previous games or in projec D. Like well I don't know how it is called in english but: - Zo een naamplaatje dat buiten hangt bij iedere winkel die je ziet in de Divinity games -. Some barrels, ropes, boxes,... you got the idea.
Hope this helps. =D