A qualified tailor can complete this work but it is not without risks. Whether BB has someone on staff who can do it is hard to know. If you have your own tailor, take the suit to him/her.
However, I believe the Fitzgerald 1818 model - the line now on sale - features flat-front pants on most or all models. It's a single-vent jacket, and you won't find much love for that around SF, but the pants are flat and slimmer than other 1818 models I've seen.