Brett produces more of its funky, earthy characteristics when it is added in small amounts in the secondary. Stressing the brett into an environment with no O2, alcohol, and minimal sugars available force it to work harder which make brett into its super-attenuative beast.
With that being said, a single pitch can easily be added to the keg after the beer has completed fermentation in primary. For best results, a FG of at least 1.010 is helpful to provide the brett with some remaining dextrins to work on. If adding it to the keg, I would try to keep the temps around 60-65F for more "cleaner" brett flavors/aromas (i.e. more fruit, less funk), and 70-75F for more phenolic funkiness. This of course, is strain dependent. Allow the brett to work for around 3-4 mos prior to taking a sample to see where you are at. 6 mos is not unheard of, and even more sometimes to develop the layered complexity you may be looking for.
BTW, which strain do you plan on adding?