I did just this a few days ago to set up a MacBook. I can't now remember why I didn't just use the MacBook Pro I use everyday. And so here's the point: Not only can you do it from a clone on an external drive, that drive doesn't have to be Firewire. My clone is a USB2 and Miggration Assistant saw it as Firewire in Target mode.
I'm not sure if the same is true is your clone is an image.
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