First off sounds fun! But I think I would finish ferment with the Brett. It always cleans out more sugars so will over carb in a bottle maybe? The million dollar question is when In fermentation to introduce it. My guess would be with just a few points left as the sour isn't a huge stand out if I remember correctly. Keep us posted.
CHEERS AND GOOD LUCK
Edit:
By the way had a beer from Treehouse brewing a while back and it had a slight sour tart to it. Liked it so thought maybe would propagate the yeast. Did a small starter and it was very sour, not like the beer at all.
They must be doing what you are investigating