Harvest netblocks of good MTAs from SPF for whitelisting from greylisting

Hi,

I'm configuring greylisting with pf(4) and OpenBSD spamd, and one of
the things I would like to do is explicitly whitelist good MTAs of
e.g. google.com, apple.com, ebay.com, schwab.com, freebsd.org,
uwaterloo.ca etc.

It seems like with the proliferation of SPF, this might be relatively
easy to do: the netblocks of many valid email servers are individually
published by each respective domain through various SPF-compliant
records.  (And even when there are no explicit "ip4" or "ip6" records
with the IP addresses or netblocks, an explicit or implied "v=spf1 a
mx" could still do the trick.)

What's left is to have a list of "good" domains, and a script that
will go through all such domains once a week to compile the list of
good netblocks.  Such netblocks could then be exempt from any kind of
greylisting, such as to never delay the mail from trustworthy domains
whatsoever.

Cheers,
Constantine.