First off, what situation landed you in such an awkward internet connection scenario? I've seen a lot of messed up configurations, but requiring customers to tunnel all of their traffic to a remote ssh server is absurd. I must say, it sounds like you're perhaps trying to run a server somewhere where you aren't supposed to, or are trying to fly under the radar somehow.
Anyway, why don't you just make another non-dynamic ssh tunnel for the smtp traffic? Something like:
$ ssh user@host -L:2525:remote.smtp.server:25
Then you can have postfix deliver use
localhost:2525 as its "smarthost".
This will be much more straightforward than using a dynamic proxy like you're currently trying to do.