Brightness application for Waveshare's 7inch HDMI LCD (B): https://www.waveshare.com/wiki/7inch_HDMI_LCD_(B)
Compiled binary takes only 422KB (Delphi 2007). Application search for HID device with VID:0EEF and PID:0005.
Possibly can be modified to handle also 7 inch HDMI LCD (C).
For RaspberryPI/Raspbian/Linux see this python script