From my experience, if you arrive at the festival on Wednesday, you're not going to have too much trouble getting some official Wacken merchandise.
My camp's managed to get exactly the stuff we want every year by doing our merch shopping during Wednesday afternoon/evening. Note that they have two big places where you can shop - they have a big site on the festival grounds and they have merch sales in the official Wacken Office in the village as well.
Typically they'll have about 5 different models/colours of T-Shirts with the same available in hoodies as well + girlie versions too.
They generally seem to be pretty ready and aware of how much they should have and what sort of sales to expect - but yes, if you pop by friday evening, you are most likely going to have some problems getting exactly what you want.
Sweet. I hope that's the case this year. In any event I'll make sure to get there early on Wednesday and grab a shirt and other stuff.
I'm guessing at the metal market there are tons of merch from bands and stuff as well? Probably a bit easier to shop for as well?
On a related note...does anyone know if US XXL mean the same size in European clothes? Kind of a strange question but I'd hate to buy shirts to find they are smaller than what I expected...or larger for that matter.