You're gonna have a tough time with this. I don't think it's so much the static IP that's an issue, because you can use a DDNS host (like no-ip.com) to give your dynamic IP a hostname (e.g., myserver.no-ip.com) that will always point to whatever IP you get assigned. I think port forwarding is the larger issue. You can read about it here:
https://helpdesk.privateinternetaccess. ... on-my-VPN-Basically, if you enable the port forwarding option, PIA will assign you a random port each time you connect.
You would then have to configure Subsonic to use that port, somehow. I don't know if changing the port is an option in Subsonic or not.