I played with this for a while and found a way to make it work.
You can load HTML directly into the webview. If you add :
<meta name="viewport" content="width="devicewidth"/>
into the HTML and then make the Twitch player half the size of device width, you can set the initial scale of the webview to 100%. The controls for the player will then be larger and the video still fits to screen.
Hope it helps someone.