This could be solved by archiving/rotating old entries, and have the webserver return the n latest.

For instance, you could let nginx do this:

  location /masukomi.txt {
     default_type 'text/plain';
     content_by_lua 'local tail = io.popen("tail -n 20 ".."/chroot/twtxt/masukomi.txt", "r")
     for line in tail:lines() do
       print(c, line)
     end
     ';
  }