Looking around the Mania thread for when KnK was selling out, seems the counter if Fate/Zero follows KnK's footsteps will appear March 3rd or March 4th on Rightstuf. KnK's appeared February 3, 2011, and it completely sold out come February 7, 2011, before the release date of February 8, 2011.
As I've read, there were 800 copies brought over here. The pre-orders opened December 13, 2010. Fate/Zero's opened December 1, 2010.
http://www.mania.com/aodvb/showpost.php?p=1897587&postcount=200
http://www.mania.com/aodvb/showpost.php?p=1897665&postcount=207
http://www.mania.com/aodvb/showpost.php?p=1898453&postcount=226
http://www.mania.com/aodvb/showpost.php?p=1900157&postcount=320
http://www.mania.com/aodvb/showpost.php?p=1900275&postcount=326
I'm going to be watching Fate/Zero very closely to see when the set sells out from Rightstuf. It possibly may not sell out before release due to various factors (Which as I'm bored, I'll list below.), but it certainly won't be available by the time the second box set containing Fate/Zero 2 comes out in the Fall.
What could hinder sales:
1. The legal streams. KnK when it came out, that Blu-ray box was the only legal way to watch the show. Fate/Zero however is streaming on Hulu, Crackle, and Crunchyroll. It was also simulcasted in 8 languages on NicoNico Douga.
2. Having to pay $370 twice as apposed to $400 once. KnK was a one-time thing, Fate/Zero is a two-time thing.
3. The exchange rate. KnK was $50 cheaper at TRSI than Amazon Japan at the time. Right now, Fate/Zero is about $30 more expensive at TRSI than Amazon Japan as the exchange rate continues to get better. Though the advantages you'll get from TRSI are the English translated booklet, and insured shipping.