Our company fabricates many of the brackets we use for corridor signs, sometimes by the hundreds. Often, a simple piece of channel aluminum mated to a flat mounting plate works just fine and can be manufactured cheaply in quantity.
We don't worry too much about hiding fasteners. A decent-looking fastener rarely detracts from a sign installation.
Sometimes called a "binding screw," this fastener is available in an incredible variety of configurations, lengths and finishes. It can pinch a "blade" sign in a bracket. This type of screw is commonly used for attaching handles to knife and saw blades. They are also common in leatherwork.
Brad in Kansas City