The examples you shared here don't require rewriterules on amazon s3 itself, the redirections all happen on the www.example.com site: so either in .htaccess or vhost conf or httpd.conf:
Options +FollowSymlinks
RewriteEngine On
RewriteRule ^/(about|welcome|contact)\.html$ http://mybucket.s3.amazon.com/index.html [R,L]
Granted, this will change the URL shown in the browser. If you want to do this without it being so apparent to the end user, then as @piers says, you will need to add a cname for your bucket on s3. and then reference/link to these files at your new http://mybucketalias.example.com/index.html address.