Thanks for the suggestion, but I don't think it would work. I have investigated Mephisto's code some more, and the problem seems to be it caches pages to disc.
So inside my "public" folder, there is a directory called "/2007/1/8/test-blog-post/comments" which contains the cached html pages previewing my comments. That's why LiteSpeed tries to help, by redirecting with an added "/", e.g. "/2007/1/8/test-blog-post/comments/" because it thinks I am looking for the directory.
But the POST with a new comment should not go into the "comment" directory, but rather be forwarded to Rails which will parse the POST and make a new comment out of it. I am no expert, but this is how I understand the issue.
In any case, I solved my issue by modifying the Mephisto code to stop caching comments to disc, i.e. I removed the troubling "comments" directory and now commenting works fine :-)
If LiteSpeed takes requests, I think something matching Apache's DirectorySlash toggle could be useful in a future release to help users turn off any unwanted directory redirects.