On Brompton the front luggage attaches to the front of the frame, which decouples the steering from the load. Many owners, and manufacturers too, adapt Brompton system to their folders because of this convenience.

As to the vertical orientation, the Brompton T-bag for that front mount extends vertically about as far as one can go without impacting the turning of the handlebars. In fact, the T-bag does not even work with all of the handlebars that are possible on Brompton, because of the interference with the steering. I started out, in trying to find a luggage solution for commuting, by converting the T-bag into a backpack. As a backpack, its handling is far easier off the bike than originally, but it is is still a very poor backpack. It is too short as a backpack, poorly profiled and its center of gravity is too far away from the body. For small loads it does not matter, but for larger loads with longer carrying times, it becomes a torture, hence the continued quest for a solution. In my opinion, the turning of the orientation for a bag that works equally well on and off the bike, at least on Brompton, is a must. There are commercial solutions that take this route, such as Brompton Freitag, but I have not seen yet any where the backpack functionality looked good.

P.S. Here is a commercial pack for Brompton, from Bromptonic, that mounts upright. Interference with the handlebars and raised center of mass are the negatives. Division into compartments and style are negatives or positives, depending on the rider, but generally lock the user into choices unrelated to the bike. Lumabag Urban Traveler and Bromfort basically work like an overloaded T-bag.