VolumeHIDtoOSC is a simple c service that allows to use a HID device under linux to send float values for volume control. By default it is configured for a cheap USB volume controller and the master volume of the RME totalmix osc control. You can change the configuration for your needs without rebuilding it.
You can find the projects sources, documentation and downloads on https://github.com/Apfelwurm/VolumeHIDtoOSC