Well, I'm a little confused - CMS widgets only show up on the content.php page anyway --- which is "home" on most sites.
That out of the way, a PHP widget to do this could probably look something like this:
PHP Code:
if ($_SERVER['REQUEST_URI']=="/" || $_SERVER['REQUEST_URI']=="/index.php") {
echo "<tags>my Html code here</tag>";
echo "more HTML\n";
} // end 'if'
"/index.php" would be the actual page portion of the main site's URL, and it's placed there in case people come to it from a link other than "/". Be certain that if you use the double quote for your echo() statement, you only use single quotes for html tag properties, and HTML entities for any quotes in the text itself.
I hope I've not confused you in return