There's nothing that will work for all crawlers. There are two options that might be useful to you.
Robots that allow wildcards should support something like:
Disallow: /*/
The major search engine crawlers understand the wildcards, but unfortunately most of the smaller ones don't.
If you have relatively few files in the root and you don't often add new files, you could use Allow to allow access to just those files, and then use Disallow: / to restrict everything else. That is:
User-agent: *
Allow: /index.html
Allow: /coolstuff.jpg
Allow: /morecoolstuff.html
Disallow: /
The order here is important. Crawlers are supposed to take the first match. So if your first rule was Disallow: /, a properly behaving crawler wouldn't get to the following Allow lines.
If a crawler doesn't support Allow, then it's going to see the Disallow: / and not crawl anything on your site. Providing, of course, that it ignores things in robots.txt that it doesn't understand.
All the major search engine crawlers support Allow, and a lot of the smaller ones do, too. It's easy to implement.
