Would you look at that, not only does it work in a shared project, after deleting the repository MakeCode seems to make a local copy of the code in the project I imported the extension in!
As you can gather from the photo, the usual sync button is gone, because the repo is too, but this means not only will games published with my extensions break, neither will projects. Projects that use my old extensions will be forced to remove the old extensions if they want to update though. This is a big find! Hooray!