Good question! For now the best way to do this is via an advanced theme. You could override the head_custom() and main_parts() functions to output the META description and extra content respectively. In your functions, check ($this->template=='tag') to see if you're on a tag page and then the part of $this->request after the slash to find the tag.
Of course, this still leaves the issue of how to store the extra information for tags. You should probably create a new table in the database to do this, then use a plugin with a page module to provide the editing interface.
BTW, in Q2A 1.4 I plan to introduce a new type of plugin module which allows you to modify the page content without using a custom theme. This would allow you to do the whole thing via a single plugin with multiple modules.