• You can use a non-master branch by going to Import URL and saying something like https://github.com/jrandomhacker/pxt-mypkg#mybranch. User note, this hasn't been extensively tested yet.

• MakeCode will generally only download files listed in pxt.json. Files in GitHub but not in pxt.json will be ignored and left alone.

More Information

• Extensions documentation
• Static TypeScript Reference