You can reserve an iPhone with an independent stockist like Cami (
www.cami.be), who are now advertising that the 16GB version can be bought from their web site. There is no guarantee that they will have received a product for the date that you arrive but if you call them and explain the situation then they will usually be helpful. I have my reservation with Easy-M but they are in Leuven and so getting to them from Brussels would require another train trip (assuming you are travelling by Eurostar). You can find a list of all the independent resellers on Apple's Belgian website
here (it's in Dutch but I'm sure you can muddle through).
Best of luck - I'm hoping that I'll get another "bite at the cherry" this week since I have separated my wife from the phone and now keep it near me. This seems like a ridiculous amount of hassle to go through for a mobile telephone.