This only works for public torrents and with PEX enabled. For private trackers an open port is required.

Wrong, tracker provide IP and port of leechers, which is just what is needed to hole-punch. It is just slower, as the seeder fetch tracker info 1 to 4 times per hour, so by the time the hole-punched connection is established, leecher has already sucked up all the data from peers with open ports.

If you want to build up ratio on a private tracker with closed port, seeding a piece of a fat 100Gb+ torrent will do.