jameswilson
Charlie seemed to indicate in that thread, there was no reason why proxypass should not do the job for you.
If it does not work as required, then perhaps lodge a bug report, I think having exchange behind sme is a popular requirement & therefore a valid enough request for developers to look at/fix, so others will benefit too.
There was another search result that also referred to changing the port on exchange, but that still requires forwarding of a different port, so may not fit your security model.
Perhaps you should/could also look at configuring your sme gateway in static IP mode, and setting up a DMZ, perhaps that will give you the gateway functionality plus the security for exchange.
That concept is mentioned in some of the search results, and sme expert installers (onsellers) often use that approach.