Someone gently clued me in that this was needed.

Here's a diff that implements the receiver side of things[0].
I'm going to implement the sender next.  Seems to work, at least with the
limited amount of testing I can do.  (I really need to get another box to run
dummynet or something on).

http://www.sigusr1.org/~kurahone/tcp-sack-netbsd-2.99.11.diff.gz

-- 
Kentaro A. Kurahone
SIGUSR1 Research and Development
"I am having a hallucination now, I don't need drugs for that." 

[0]: net.inet.tcp.sack=1, so it'll send sacks along on dupacks.