"As long as there's no way for me to simulate a built-in browser in a web app, HNmobile will remain being a non-standalone app."
I'm not sure if you had a look at it yet but there is a PhoneGap/Cordova plugin called ChildBrowser that may do what you want here. I can help you getting setup with a PhoneGap project for your app if you want!
"Since it's hosted on another domain, the API supports CORS so that HNmobile can do cross-domain XMLHttpRequests."
Why don't you use JSONP?