3RD Party GUIs?

Unity Engine
1

Is it possible to use flash to design the guis and then utilize them in Unity via scripting?

2

Yes, Uniswf.

3

Alternatively if you want to do it directly in ActionScript, there is Scaleform from Autodesk which offers a unity binding on all platforms thats possible (Win/OSX require Unity Pro, on Android / iOS it works on both licenses)

4

You can get more info on Autodesk Scaleform for Unity and a free 30 day eval here: http://gameware.autodesk.com/usage/casual