You can assign the IP address for the Obi to always be the same. I am not at home right now, so I can't tell you where it is. I can say that if you found the DMZ part, you should be able to find that pretty easy. It's in the advanced screens. It's pretty easy to see and figure out.
As far as putting the DMZ on a particular port on the back of the router, I personally have never seen this. On all consumer routers I have seen, it is always assigned to a specific address. That is why you can add a specific device a specific IP address. I would not want to only have the option to tie the DMZ to a specific port. What if I had another switch attached to the router (like I currently have)? If I put a hardware port as DMZ, I would have to choose one that is only that device. That is not very convenient to me. It removes a lot of flexibility.
Anyway, I was kinda hoping an Obi employee would see this and address this issue for us. They would be the ones to answer it as far as I can tell.