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Consider why you would want a password protected folder in your dock anyway? It's purpose is for convenient, one click away look at the contents inside a folder. Putting a password completely defeats the purpose of this easy access.
 
A stack is just a folder. There is nothing special about it apart from the fact you have a shortcut to it in the Dock. You can't password protect a folder so the same is true of a stack.
 
I'm sure there are password utilities...

but for sensitive data, I just create an image with 256bit-AES built into it...

If thieves steal my macbook pro, at least they won't see my important documents and work data :)
 
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