on cron, this module would look through all menu links and find all external links
possibly queue up links to get
download favicon from site root
cache them (using ImageCache?)
Similar ideas
https://drupal.org/project/link_favicon_formatter is a called Link favicon formatter
http://drupal.org/node/535948 issue to add favicons next to links using the Link module
http://drupal.org/project/urlicon URL Icon project (not available for 7 yet)
APIs for grabbing favicons: