Charles,
Can DLLC be used to provide a standard Windows callback function to deal with messages from common Windows controls? I know all of our testing with callbacks has been using IUP but it seems that concept is generic enough to use with the Windows API in general. (like your SAPI example)
If you have a moment, could you show a DLLC example of a Window with a close button and label saying "Hello World" and the DLLC callback handling the Windows messages for the Window?
John