Update 2 : I’ve written another post on how to do this without any other plugin : How to assign an icon to a page
Update 1 : as the plugin Page Category Plus is not available anymore, nor supported, it’s no longer possible. But You can always contact the author.
The plugin is capable to not only assign an icon to your posts, but also to your pages and subpages.
To do that, you’ll just need to download the Page Category Plus plugin and assign a category to your pages/subpages. Then you’ll be able to manage icons for your pages/subpages from the category icons plugin. Don’t forget to add the get_cat_icon() code to your page.php file.