Hi Johannes,
I have now run into exactly the same problem as you describe here 
. I am building a demo and I have written a SAPUI5 based application and now I want to launch that app in a new window from the launcher in the mobile portal. I created a URL iView for the UI5 app and placed that iView into the launcher folder of my Mobile Portal role.
When it launches however it does so in a new window but the width is all wrong it looks like this:
But when opened directly by a link it looks correct, like this:
This is very frustrating 
.... to me the problem seems to stem from the fact that when using the URL iView the actual SAPUI5 app is being wrapped into an iFrame and the new window that opens is actually opening the iView that wraps the app, and not the app directly. It looks related to this issue: http://stackoverflow.com/questions/15600914/viewport-meta-tag-in-iframe where the meta tag for the viewport isn't being recognized since it is wrapped in the iFrame.
So now we both need help... this is serious 
... I would like to reach out to Aviad RivlinDoron Lehmann and Dvir Cooper in case they can help us. Maybe we are just doing something silly.
But I think this will be a very common use case where people will want to open a mobile web app (SAPUI5 or other) in a new browser window on a mobile device - to make the most of what screen real estate there is on the device (if you embedd an app in the portal framework you are really left with very little space). The launcher in the PoD seems like a good way to do this but simple things like this need to work easily.
Hope you don't mind me hijacking your post, but I think we have exactly the same problem, better it is all discussed in the same place.
Thanks,
Simon