coachm - 5:30 am on Nov 30, 2012 (gmt 0)
A simple 404 > header redirect on all pages, redirect 100% to the homepage and put up a "new site" message and nofollow link to the new site home page, to avoid any good or bad history following to the new site. (would using a deep link to the new-site category page be a bad thing?)
That initially sounded good to me, except that 404's can exist in google's database for..well, like forever, because it doesn't mean the page is gone, just not found. Goog doesn't know anything more so it continues to check, so the fastest way would be to do a 410 gone redirect, I think.
I suppose, now that I think about it, that a custom 410 redirect would work best, provided you give information on the 410 page, much as you were thinking you would do for the 404.
Unless I come across a good argument for using 301's I'd think I won't use that. I don't care, for this particular site, whether it passes link juice. It's kind of an outlier in terms of topic, so it has very little value to link it to my remaining sites.