Yeah, brett can really dry out the malt character especially in a low finishing gravity beer to begin with. Berliner's don't have much in the way of malt character anyway and they will get more sour if left in the bottle or the keg with proper temps. You might want to consider adding the brett at bottling time to give it a more subtle effect (until it takes over later).
I kegged my latest berliner about in early January and although it was tart, it just wasn't sharply sour/acidic enough yet for my liking in terms of a proper berliner. It is now just sitting warm in my basement hopefully souring a bit more (which I am sure it slowly is). As much as I would love these beers to be quick turnarounds, I have always found them to be better with about 6-10 mos under their belt to develop the proper acidity needed.