A truly bespoke suit should fit you even if you're not wearing a belt. So skip the belt loops and go for side tabs, with a few buttons on the side of the trousers that allow you to adjust the waist (you may be wearing this suit to a very fancy lunch, after all). You get a clean, uncluttered look around the waist.
Interior buttons for suspenders are also an option if you're ready to really commit to an old-time look. It should go without saying that novelty suspenders are a very bad idea unless you're working the vaudeville circuit.