Q:

Reverse proxy function

Question about how a reverse proxy appears to the requester:
If I have a secure web app running on a port lets say 9443 and a customer who due to corporate IT restriction can not access it and I use a reverse proxy to act as a DMZ and forward 443 traffic to 9443.  Will the customer be able to access the web app. 
Please assume that changing the app on port 9443 is not an option.   

A:

Yes, you can use something like apache/nginx to proxy your requests from a DMZ to the backend application server running on that port.   You just need to make sure the ports are open from the DMZ to the LAN where the appserver is.