Not really the solution you asked for, but here's one way to work round it without spending a lot of time messing with config's, it also has the advantage of being less prone to breaking after updates and upgrades.
2.1.3 The Default Virtual Host
If you attempt to access a domain that Pow doesn't understand, like http://localhost/, you'll see a page letting you know that Pow is installed and working correctly, with instructions on how to set up an application.
You can override this behavior to serve all requests for unhandled domains with a particular Rack application. Create a symlink in ~/.pow named default and point it to the application of your choice.
Add to that the information from here:
2.4 Serving Static Files
Pow automatically serves static files in the public directory of your application. It's possible to serve a completely static site without a config.ru file as long as it has a public directory.
And that should create a default Rack app that can serve up content based on IP address (no host headers)