Here's how to achieve what you need through an IIS URL Rewrite:

<rewrite><rules><rule name="ForceLowercase" stopprocessing="false"><match url=".[A-Z]." ignorecase="false"><action type="Redirect" redirecttype="Permanent" url="{ToLower:{R:0}}"></action></match></rule></rules></rewrite>

Translation: If the URL contains any uppercase letters, convert it to all lowercase, then continue processing other rules.  (You can name the rule whatever you want.)

These rewrite rules are stored either in the ApplicationHost.config file or in Web.config files. More info and step-by step instructions: creating IIS URL Rewrites.

Note: Compared to the [ISAPI_Rewrite] filters (httpd.ini), I've found that the IIS URL Rewrite (web.config) rules seem to match the syntax of Apache more often. As a result, it's easier to find RegEx expressions already written for what you need. It also helps to focus on debugging the actual code versus some strange IIS exception. Good luck!