We have such a class at our university. We used to have students issue PRs to real projects but then stopped for that very reason. Now we have our own big OSS project that they work on quarter-to-quarter. Seems like a decent compromise.

Oh, since you're here, I want to say thank you for all your guides! I learned from them yeaaars ago, and still recommend them to my own students to this day, especially your network programming guide which is linked from all of the network lab session sheets of my systems and networks course. Thanks!