If you're using a db for the maps on your postfix server then it shouldn't be too hard.
- Give your postfix db user permission to connect from the plesk box.
- Write a shell script (or scripts) on plesk box for modifying the postfix db on external mail server.
- Add the event triggers in plesk for mail account created, mail account deleted, etc.
You will also need to have the shell scripts modify the postfix maps on the plesk box directly in order to have it appear that mail is enabled on the plesk box even though it's really using an external server. I've only done this with plesk running qmail but I don't see why it wouldn't work with plesk running postfix.
This is exactly what plesk expand is supposed to do. Unfortunately expand isn't a viable option any more. If you try this out then post back here and let us know how it went.