WebApp Integration with login redirect to my service's OAuth2

I'm implementing a web app integration that looks like the "Send to Chatter" integration. User clicks on a file, selects my integration and I want to redirect them to a login to my application before processing the selected file.

It looks like the page referenced in the "Client Callback URL" field in the app is wrapped in an iframe. I can't redirect from there to my login page to start up an OAuth flow. 

Are there any examples of capturing my callback parameters and then redirecting to an OAuth2 flow?