How can I protect hardwood floors from a deep freezer defrosting?
I am sure they sell large pans somewhere but that shouldn't be a concern.  Your freezer should be contained, in that if there is a power outage and everything melts - the water should stay in your freezer.  

Note:  I have to think if I was putting a deep freezer on my hardwoods I would lay it on an area rug.  Even insulated the freezer bottom is pretty cold comparatively so I would try to bridge that temperature gap.  I would not worry about spillage though.  Although you could test this before bringing it in your house with a hose.