How much are you offering? I downloaded by the by-date archive page and wrote this Python code to download the individual message pages, with the thought of catting them together to get your "single file." (Which is an ugly hack and not even technically valid HTML, but it worked when I've done it in the past.) But it's only halfway through downloading and I need to turn the computer and its noisy fan off and go to sleep now; this should be easy to finish tomorrow.

import re
import requests

# http://sl4.org/archive/date.html
with open("sl4_archive_date.html", 'rb') as toc:
    archive_page = str(toc.read())
    results = re.findall(r"http://sl4.org/archive/\d+/\d+.html", archive_page)

for i, result in enumerate(results):
    message = requests.get(result)
    with open("messages/{:07}.html".format(i), 'w') as f:
        f.write(str(message.content))

I suggest looking at hypetombox.pl, which should convert the archive into a Unix mbox file. You probably want to use wget first.