XUL Booster is a third party Eclipse / WebTools build that offers some support for XUL Editing and awesome support for extension packaging.
Extension Quick Start
XUL Booster makes creating and packaging an extension very easy. It supports exporting XPI files or registering a file system based project.
Create an extension
You'll need a GUID; try the Online GUID Tool.
Add an Overlay
You'll need the chrome address of the content you wish to overlay, ex. chrome://browser/content/browser.xul
To create an XUL:overlay, create a XUL fragment with a top level node that has an ID matching an existing element, ex. menupopup id="menu_ToolsPopup"