An update: I have started this project! Measured in scraping time, it's the biggest project I ever started. In the past 9 days, I've scraped about 4% of all data, so I expect to complete this around August.
There's also a chance I'll run out of disk space because of the millions of large posts made by bounty spammers, but I'll deal with that when it happens.

Sneak preview: http://loyce.club/archive/oldposts/
How to use:

• Find the msgID you need. Let's use 28228
• Remove the last 5 digits from the msgID to get the directory name (if there are less than 5 digits, use 0): 0
• Replace the last 2 digits of the msgID by xx, and add .html (if there are less than 5 digits, use 0xx): 282xx.html
• Add "#msg" and the msgID: #msg28228
• Put everything together and go to http://loyce.club/archive/oldposts/0/282xx.html#msg28228

Limitations

• Currently, the first 2.1 million posts are available.
• I'll scrape the first 5.21 million topics and all posts in there.
• That means I'll archive 53.36 million posts, this partially overlaps with my scraper for new posts.
• This is a one-time thing, I won't update it with newer posts (I scrape unedited versions for those).
• The time "scraped on" is Amsterdam time.

If no username is mentioned, it's either "Anonymous" or "random". I forgot those exist when I started scraping, and it's not important enough to start over.