Not sure about Pico B, but I'm pretty close to making it work on Pico Pro. Here's what I did:

-compiled buildroot with UVC gadget (you can also manually edit /etc/init.d/S50usbgadget.sh to enable UVC mode)
-cross-compiled uvc-gadget from https://github.com/wlhe/uvc-gadget
This version of uvc-gadget ...