I have a first example using the Kendo Toolbar widget and I was able to get an Angular button rendered in the toolbar:
http://dojo.telerik.com/ucIfin
When I tried to do the same thing in the Kendo Spreadsheet widget toolbar, it failed to compile my template:
http://dojo.telerik.com/UseWI

In general Angular directives are not compiled properly when specified in the template of a Spreadsheet Toolbar item.  I am aware of a workaround for the kendo-button specific case, but I would like the template to work for custom Angular directives as well.