I am planning on a doing something similar this weekend and have been researching this topic a bit. I would say it depends on what you want? If you pitch the Brett after primary, it should produce more of the "barnyard" phenols, whereas pitching earlier I think gives off more of the "fruity" flavors. Given that the blend you have has so many different strains, I would vote for letting primary finish, then adding the brett and letting it go for a while to get some funk (which Im partial to).
For the record, my "Brett Saison" is going to be a 10gal batch, 5gal is getting WY3711 + Brett B co-pitched. The other 5gal is getting WY3711 in primary (with 2 gallons of the 5 being kettle soured x 2 days, then added back to primary when finished souring) and then Brett B in secondary. I know the kettle souring throws a little bit of a wrench in the comparison between the two, but Im hoping that there will be enough of a difference between the co-pitch and the secondary with brett to learn the differences in timing of the brett addition.