Couldn't think of a way of doing this through IIS as easily as editing the configuration for WebSphere.
I don't use WebSphere, but the docs suggest it's possible to include or exclude UriGroups using Route rules:
A group of URIs that will be specified on the HTTP request line. The same application server must be able to handle the URIs. The route will compare the incoming URI with the URIs in the group to determine if the application server will handle the request.
Following is an example of a UriGroup element and associated elements and attributes:
And these look like they're used with the routing rules:
Using the information that is defined in the VirtualHostGroup and the UriGroup for the route, the plug-in determines if the incoming request to the Web server should be sent on to the ServerCluster defined in this route.
Following is an example of this element:
<Route VirtualHostGroup="Hosts" UriGroup="Uris" ServerCluster="servers/>
So I'd go that way - the IIS alternatives involved setting up other websites and ARR and routing rules and... it's just messy. Exclusions are probably simpler!