Ever since we've introduced the Extension Builder toolkit, we have been asked for (even) more control over the behavior of extensions.
In the download link below you will find a document which describes a method of implementing your own extensions through COM.
Examples for VB6, VB.NET and C++/MFC are included.
This project is still experimental, it's not officially supported.
FOR ADVANCED USERS/PROGRAMMERS ONLY.
Download link:
http://www.wysiwygwebbuilder.com/wbx_com.zip