I understand that you would like to know how to create an Annotative Scale for BlockReference using ARX.
This question may be better asked at the ObjectARX forum here on the Discussion forums: Autodesk - Discussion Groups - ObjectARX
Also, look through the Customization information at the Autodesk Product Support link below: Autodesk - AutoCAD Services & Support> Customization
Our "Learning Paths" tutorials contain much information on how to use the program, what the basic options are, etc. and will get you well on your way to creating your drawings. Learn the Basics and Additional Resources
While researching your question, I found the following link which may have some helpful information that can help you create the ARX you require: ARX - hot downloads about keyword ARX
I hope this points you in the direction where someone can better assist you with the customizations.
Autodesk Product Support