Thank you to Alan De Windt who has submitted news about the release of his latest SDR# Plugin called "SDRSharp Controller". Alan writes that this is a plugin that is "similar to the existing SDRSharp Net Remote plugin by Al Brown but which allows simpler physical controllers to be built".
With this plugin you can create a key/value text mapping to turn any USB control device into something that can control various settings in SDR#. The controller hardware could perhaps be anything from a USB knob controller to a gamepad.
Alan also provides an example of a hardware USB knob controller that he's created which works together with the plugin. On the linked page he shows the components required to build the controller, how to wire up the circuit and provides the Arduino code.