Codex tools: Log in
The "page_template" filter can be used to load a custom template for a given page. It will replace the template used whenever the "page" template is called.
A plugin can register as a content filter with the code:
<?php add_filter( "page_template", "plugin_function_name" ); ?>
Where "plugin_function_name" is the function WordPress should call when the content is being retrieved. Note that the filter function the plugin defines must return the a full path to a template file or the resulting page will be blank. The template file should have the same entries as the "page.php" file has in the theme.