I'm just getting into Joomla, but you could theoretically insert some JavaScript into individual pages using the Article editor. Of course, you'd have to do that on a page-by-page basis. The alternative would be to edit one of the template files to do this on a site-wide basis, but that's beyond my experience.
On the other hand, unless your user base is very unusual, only about 1% to perhaps 2% of your users are likely to be using IE7. So I don't think it's worthwhile to put any significant effort into catering to them - even to advise them they should upgrade or use a different browser. By now, those users should be well aware that IE7 has significant limitations, and that virtually all of the problems they encounter online are due to issues at their end of things.
