Hello,
Unfortunately, we aren't going to get much of a chance to test Ext.NET with Nancy. Although after releasing Ext.NET 3 we might take a look.
My guess is Ext.NET + Nancy is not going to work. There's a pretty big dependency on WebForms and ASP.NET MVC in the assembly, including conditional compilation and MVC specific inheritance requirements. That might be tough to work-around for Nancy.
It should be possible to get it working, but will require some testing.
If others would like to see Ext.NET + Nancy support, please voice your opinions here.