I wanted a volume knob. It was easy enough to hook a rotary encoder and use the NicoHood HID library to get the keys and send them to the host. I then wanted a led to show the volume but didn't want to write my own protocol for it so I found the Blink1 but I didn't want two devices where one would suffice so I hacked the code for the blink1 protocol to work on my Arduino Pro Micro (originally for an attiny85) and integrated that with the keyboard output.
I got the basics up and running, I integrated the blink1 protocol with the keyboard and they work. The blink1 protocol is only partially implemented but the rest is only porting the code to get it fully implemented in the arduino environment rather than bare attiny85.