Just occurred to me, and I realize that a lot of developers have integrated their own solution for this since there are forum threads going back to 2009, but why doesn't Ext.Net have a built in GoogleMapPanel with a few apis like adding markers with clickable tooltips?
http://www.sencha.com/blog/integrating-google-maps-api-with-ext-js/
https://github.com/VinylFox/ExtJS.ux.GMapPanel/