This is already possible although I wouldn’t expect it to be expanded too much for vanilla. With the current vest in-game you can add extra storage to either the left side, the right side, or both sides. It appears to be a feature that will only be used for specific clothing items though. If you mean moving the pockets freely then I don’t think that feature will be put in-game.
That isn’t really what he meant. He meant moving the attachment itself, not the entire rail.