I do not see any way the seed of the nephilim could have survived the Noah Flood. If Nimrod were exceptionally tall (and I'm not aware of any ancient manuscript supporting that view) then I'm confident it would not be because offspring of the Watcher-angels and mortal women physically survived (Genesis 6-7)
Also, the only reason I can perceive why Canaan was singled out in the Noah curse would be that Noah was speaking prophetically, i.e. God sees the end from the beginning.
Deuteronomy 29:29 (NIV) 29 The secret things belong to the LORD our God, but the things revealed belong to us and to our children forever, that we may follow all the words of this law.
1 Corinthians 13:12 (KJV) For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.
I like these verses when I do not know, as my tag line says
I do not know but, Thank God, God knows
I am going to drop my anal questions about demon seed for a while and let God reveal what He wants me to know