Is it possible to create a traditional Revit Add In, having its own place on the addin tab or its own custom ribbon tab, using DesignScript as the language instead of C#?
I guess not.
You could always make that suggestion here though 
You can use Dyno to do this. Works with keyboard shortcuts too.
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I have requested a similar ability here:
http://forums.autodesk.com/t5/revit-ideas/dynamo-script-run-buttons/idi-p/6331443