I have had some wonderful ciders, especially those from Farnum Hill in Lebanon NH that all seem to have some barn yard to them. Or as Steve Wood (the owner of Povertly laneOrchard/Farnum Hill cider) calls it, "FYM or farm yard manure" character. That would seem to me to indicate some strain of Brett. Sadly I never asked him that when I took part in a cider workshop that he was also participating in.
I have thought about this as well as I would love to add that characteristic. I have only been making cider seriously since last season. I made 25 gallons last fall. Fifteen gallons where from cider I pressed myself. The other ten gallons of juice came from an experienced cider maker, Terry Bradshaw of Lost Meadow Orchards here in VT. The latter turned out excellent. Nicely balanced, flavorful, good tannins. The former... Well I did 5 as a New England style cider using raisins and brown sugar to fortify. That turned out decent, nothing wonderful but drinkable. Fairly tart but with some character at least. The other ten, well... Extremely tart, dry, flavorless, and wicked pale.
Anyway, sorry about the rambling. I have been pondering this myself as I contemplated pitching some Brett. dregs into one of the kegs of the lackluster cider.
The problem is, cider finishes bone dry most times left to its own devices. Unless you do something preferment like keeving, or during ferment like filtering or using potassium sorbate. That means, the Sacch. will eat everything before the brett. has a chance.
So, if you want a brett. character you need to do as a alikocho does and rely on the natural microflora of your apples and not use sulfites. Or... I don't know. Maybe if you sulfite your juice and wait the appropriate time then pitch a culture of Brett. it may work. The idea of sulfiting is to knock most of the wild yeast and bacteria out before you pitch your selected yeast culture to give it the upper hand. The trouble with not sulfiting is you really have no control over what ends up fermenting your juice. You could luck out, or you could end up with a slew of undesirable bacteria and wild yeasts that will give you an undrinkable end result.
The tartness you mention desiring can be a component of cider from the apples themselves. If you choose apples that have some acidity (tartness) to begin with it will manifest itself in the finished product more as the sugars of the apples will disappear and the acidity will remain. This was my problem. I used too many abandoned/feral apples that I found on the roadsides and they were wicked tart to begin with. The fermentation process only exacerbated this.
The key to cider is blending unless you can find some naturally balanced cider varieties such as Kingston Blacks. Otherwise you need to combine different apples to give you the proper balance of sugar, tannins, acidity and flavors.
So, really the only thing brett and other wild yeast can positively contribute is other flaovr components such as some barnyard sweaty horse blanket and such. How to utilize them predictably... I am not sure myself yet.