I'm really excited to see this post! I've actually tried doing something similar -- I want to use/create a smartcard reader on the web that works with gpg.

The goal was to port pcsclite or scdaemon to use webusb instead of libusb.

It's been a while; I forget exactly where (very early) I left off/felt defeated. I definitely didn't consider making an additional backend for libusb!

Very cool -- I can't wait to see part 2 ;).