There should be a hook that other module could implement to display tagged content on /tag/index.php, without having to modify the file.

We could add a [modulename]_tag_section(string $current_tag) hook. (Would that be a good name? Suggestions welcome...)

The function would pass the current tag as an argument, and expect a fully formed html string, to be displayed.

This could be a good occasion to get the user- and blog-specific code out of /tag/index.php and into the related lib file..