To see which places in the Fisheye/Crucible UI allow web items, enable the Development Mode Plugin. A placeholder web item will appear at each web item location.
To enable the Development Mode plugin:
All web item locations will now show up as you browse Fisheye and Crucible. Each web item location will now have one placeholder web item with the text 'Plugin Point', in bold and red. Hover over the link to find out the key to use for that web item in your atlassian-plugin.xml. The placeholders link to the web item documentation.