Remember that technology exists, usually for some time, and often in a completely usable state before it makes it to RELENG.

http://2002.eurobsdcon.org/papers/zec_presentation.pdf

Hell, you can even still download the freebsd 4 patch from archive.org:

http://web.archive.org/web/20030523110842/http://www.tel.fer...

It's all over mailing archives from that era, too.

I mean, I'm not saying I'm a kernel programmer... but I'm not an idiot; I can apply out of tree patches.