A customizable scheduler for real-time computing is a typical example of the advantage of extensible operating systems. We have applied the reflection technique to a traditional operating system and experimentally implemented a customizable scheduler for real-time computing. This paper reports this experience and discusses the limitation we encountered. Then this paper presents a new mechanism we call weak protection and mentions that it is useful to make a reflective operating system workable. We are currently developing this mechanism for our DECADE operating system.