While Sebastian's code gets the job done, I believe it doesn't completely match the suggestion -- for example, it requires declaring a factory delegate and registering it in the container.
So, if anyone is still interested in this suggestion, I implemented a Unity extension, which, I believe, matches the suggestion exactly and also is more in line with the Unity's parameterless automatic factories experience.
While Sebastian's code gets the job done, I believe it doesn't completely match the suggestion -- for example, it requires declaring a factory delegate and registering it in the container.
So, if anyone is still interested in this suggestion, I implemented a Unity extension, which, I believe, matches the suggestion exactly and also is more in line with the Unity's parameterless automatic factories experience.
https://github.com/mykolav/unitycontainer-param-autofactory