I'm setting up a client's firewall, and he would like to use OWA. I currently am only allowing OWA only over HTTPS with a certificate required. The problem is, I have other services I wish to expose using HTTPS. The firewall I've chosen (ZyWall 2 Plus) allows port translation. So, I have a firewall translation rule that says convert incoming 8500 into 443 on machine 192.168.1.2.  When I look at the firewall logs, the incoming: https://mysite.com:8500/exchange is being interpreted as 443. It (or by default because its https) is ignoring the 8500. Is there a way to run https over a different port, or am I pretty much done? My only other thought would be to register different romain names & have IIS trigger on the site name requested (ex: mail.mysite.com).

You can use any ports you want. Anything 1024 or higher is a non-standard port and can be used. Up to 1023 have protocols in use.
 
I'm not sure of your configuration but it looks right.. the :8500 signals to use port 8500 but how is your website hosting setup? Are you doing it?  Make mysite.com/exchange switch to port 8500 instead of 443 or 80.