Oh, supporting files. I assume you're not editing the individual pages, so there's nothing you can do to stop the end user's browser from requesting "école.jpg" in the first place. But can we assume you've got encoding information in place for the page content, so the browser is requesting é rather than Ã©?
There are two points of contact: What the browser originally sees, and what reaches your server. In between is a vast area that's out of your control. What you may need to do is set up a simple RewriteRule that looks for any URL containing \\x (escape the literal backslash) and rewrites to a php script that does the conversion. Note that \x is essentially the same as %. The form has a technical name which escapes me haha at the moment.