is it possible to expose C# delegates and / or events in the Editor?

The title says it all. Basically,
say I have:

public class MyClass : Monobehaviour {
   public delegate void handler(object sender, EventArgs e);
   // ...
}

Can I somehow expose handler in the editor and be able to add delegates to it from the editor?

No you can’t. Unity doesn’t serialise delegates.

The best way I’ve found to do what you want is:

abstract class Handler : MonoBehaviour or ScriptableObject
{
    public abstract void Handle(object sender, EventArgs e);
}

And that doesn’t even work for many of the use cases I’ve needed delegates for.