Instant Web Publishing runs through the web server, so to change the port you'd change the port your IWP site is running on on the web server. This can be done with the OS X Server Admin application if you're using OS X Server.
So, to get IWP to run over 591 (for example), would I need to set up a new "site" in Server Admin that runs over 591?
I need my regular stuff to run over port 80 still...
(Is there documentation anywhere for this?)
Any pointers would be appreciated!
Yeap, you'd make a new site running on port 591. You can make it the same domain as the one running on port 80.
I don't know of any specific instructions on doing this, other than the OS X Server documentation.