They may block port 25 for dynamic IP's and domestic customers but I doubt very much if they would block it for business customers.
The only other ways I can think of doing it are by running a cloud instance of postfix e.g ClearOS and then relaying to your main version of ClearOS either in a VPN or by switching ports. ClearOS allows you do to Mail Forwarding to different ports. You'd then need a port redirection rule in your main firewall to switch back to 25 or you'd need a separate instance of postfix running on a different port (harder to do). Easier in some ways would be to use the VPN route between the cloud instance and your main server. Then no port switching is needed.