Agree, you aren't going to get sourness from brett alone and you only need a few drops from the lab packaging per bottle.
Bottle bombs could be an issue especially if your FG is fairly high. A couple insurance policies would be to use thicker glass bottles and to add like 3/4 the priming sugar and let brett's continued fermentation provide the rest of the CO2.
Wyeast Lambicus would be a good addition to a dubbel if you can find it.