If you already performed checkout for the repository (http://community.jboss.org/message/575216#575216) then the path for JBoss HTML editor would be <Your trunk folder>/jst/plugins/org.jboss.tools.jst.jsp/src/org/jboss/tools/jst/jsp/jspeditor/JSPMultiPageEditor.java
You have to import org.jboss.tools.jst.jsp plug-in to view/edit some code. But, IMHO, you need to create your own plug-in and your own editor extending JSPMultiPageEditor.