I noticed this in Loader.js:
var isNonBrowser = typeof window === 'undefined',
    isNodeJS = isNonBrowser && (typeof require === 'function')
How about having the property isNodeJS available directly as a property of the Ext package, in order to use code like:
if (Ext.isNodeJS) {
   //Server-side specific code
That would be of a great help to determine, e.g., which proxy should be used depending whether you're in the server or in the browser.

I'm glad to see that Sencha has some plans about Ext being used on the server side as well! I can't wait to use a full and consistent MVC stack both server- and client-side.