In my previous theme I made some code for this, though not in plugin format.
I not only wanted to add some text but for specific tag pages other theme layout, other sidebar, other header, etc...
For this I made the structure to make directories /inc/tags/ in the theme directory.
Then ... there is quite some code but e.g.my tag.php looked liked this:
<?php
$tagname = get_query_var('tag');
get_header();
$tagdirectory = TEMPLATEPATH . '/inc/tags/' . $tagname . '/';
$includedirectory = TEMPLATEPATH . '/inc/';
include ($includedirectory . "directorytag.php");
include ($includedirectory . "header_top.php");
?>
<div id="maincontent">
<?php
if (file_exists($tagdirectory . "header.php")) {
include ($tagdirectory. "header.php");
}
?>
<?php
if (function_exists('postbar')) postbar();
include ($includedirectory . "loop_tag.php");
if (function_exists('postbar')) postbar();
?>
</div>
<div id="sidebar">
<?php
include($includedirectory . "sidebar_tags_top.php");
if (file_exists($tagdirectory . "sidebar.php")) {
include ($tagdirectory . "sidebar.php");
}
include($includedirectory . "sidebar_tags_bottom.php");
?>
</div>
<?php get_footer(); ?>
The idea behind it was like this:
I think someone who just starts with blogging without a real purpose will blog along... and during the years he will discover that he wrote quite a lot about some things indicated by a greater amount of tags. So these more interesting tags give both some insight in his interests and lead to kind of subsite based on these tags. So very "evolutionary" as opposed to "I have these interest and will start a wp multisite on these topics".
So (technically) : as soon as you add a directory under the /inc/tags/ directory with the name of the tag you can add files like "header_top.php", "sidebar_tags_top.php" etc... and... if a certain tags has really specific functionality you can also include it in the loop to show up on the page. You could even add a total different application in the tag directory as time goes by. Therefore: since many combinations are possible this is not really pluggable.