rish3 - 4:35 am on Aug 4, 2013 (gmt 0)
One approach would be to send back a 401 "access denied" for known search engine spiders, but a 301 back to everyone else.
Of course, Google could decide that's cloaking. Perhaps customizing the 401 page to say "we do not allow search engines to crawl this website. However, we do provide a 301 redirect for real human visitors, that takes them to the new location of our website".
That seems like something designed for real visitors, and not for search engines, which Google says it likes. Still risky though.