Some of this work is done here and adapted/ simplified here. This is an I2C interface but the encoder and button code could be used. There are some HID libraries for Arduino. I am using STM32 hardware and the Roger Clarke libraries which I started to detail here. The idea of a USB keypad was discussed here..
I quite like the idea of a USB control pad but there is benefit from having the knobs close to the relevant part of the screen. Of course that loses some benefit when the screen is 55" and the knob is 20mm. (I couldn’t fit another measurement scheme in that sentence 
. )