Hi friends, excuse me if I was a bit absent but have been very busy with work. I was able to write a first version of the USB-MIDI drivers for Attiny4313 (for nanoDrum, but can also be used separately). Here is the link on GitHub. The code is not yet fully operational, but I hope to finish soon so I can stop the fundraising and send the first nanoDRUMs.
picoMIDI is based on the v-usb but works through ArduinoIDE, for now I will not write a guide to use it separately, but you can follow this link. Other references you can find them in the file picoMIDI.h.