Behold! I have the answer, at least if you’re willing to use Qbittorrent and the PIA app: www.reddit.com/r/…/jr2mptg/
This is an AutoHotKey script that runs at boot. It basically babysits the Qbittorrent port for you, making sure it matches your randomly assigned PIA forwarded port. I’m guessing this is also possible using a different VPN or torrent client, but you’d have to find your own script for that. Also, make sure to bind Qbittorrent to your VPN interface: reddit.com/…/guide_bind_vpn_network_interface_to_…
P.S. I should point out that port forwarding on your VPN isn’t TOTALLY necessary…but if you don’t forward ports, your download speeds will be slower and you’ll barely be able to seed at all (this is known as a “dick move” and it will get you banned from private trackers).
P.P.S. I should ALSO point out that your forwarded port does not change very often if you keep your VPN connected! So you don’t actually NEED this little script, you could just check the port once a week and make sure that Qbittorrent matches it. For example, I’ve been on the same port in PIA for at least a month now, and I’ve definitely rebooted my PC a couple times for updates. So it seems that the PIA app often just reconnects to the same server and uses the same port, but YMMV.