To offer new sidebar content to Composer users, you have to distribute your application as an installable extension to Composer, a *.xpi file.
The application id for Composer is
Your extension should add elements to chrome://bluegriffon/content/xul/bluegriffon.xul through a XUL overlay :
nameattribute, representing your sidebar content ; that sidebar should be a NAME ; it is recommended that the ID and that attribute have the same value
srcattribute, containing the URL of the document to be shown in your sidebar content; this is typically going to be a chrome URL in your XPI
titlecontaining a (possibly localized) string representing the sidebar item. That string will be used in the sidebar to list the available sidebar items or title the visible sidebar items. If absent, the value of the
nameattribute is used.