I believe this is not possible as is.
However, with just a bit of JavaScript, you can get a custom "intro" to show up on each tag page, but the intro would be hard to customize because it would need to be written into the theme code.
If you really really want the intro content to come from a static page, you're probably going to have to work with XMLHttpRequest or iframes. Actually, I think there's a fairly elegant way that makes use of an iframe and iframeElement.contentWindow.document.body.innerHTML.
Are you willing to have the intro text show up only when people are using JavaScript?