Something

$query = "SELECT nick, hstring, avg(score) avg_score
FROM
  (SELECT channel_lines.nick, channel_lines.hstring, channel_lines.score
  FROM
 (SELECT DISTINCT nick, hstring
  FROM searchy
  WHERE channel = '##windows'
  ORDER BY increment DESC
  LIMIT 20
 ) AS nicks,
 (SELECT nick, hstring, score FROM searchy WHERE channel = '##windows' AND mdate > $datestop) AS channel_lines
 WHERE
   channel_lines.nick = nicks.nick
    AND
    channel_lines.hstring = nicks.hstring
 ) AS last_lines
GROUP BY hstring
ORDER BY avg_score";