Update - it works ! At least I can connect to Mountain Lion (not server) from my iPhone using the VPN Server Configurator app.
Here's what I did :
1) download the app and install
2) setup using the help files on the web page : http://www.greenworldsoft.com/product-vpn-server-help.html
3) at the last stage you need to setup port forwardin on your router
4) under Airport Utility 6.0 you cannot setup ports 500 or 4500 due to BTTM conflicts but setup the other 2 ports (1723 TCP and 1701 UDP), update airport extreme
5) download Airport Utility 5.6 from here : download already extracted utility it is in it's extracted form as is necessary under Mtn Lion (thanks to NetUse Monitor for the download - great app by the way)
6) run 5.6 and setup port forwarding (Advanced-Port Mapping) for the other 2 ports (500 and 4500 UDP), update airport extreme
7) that's it, I was able to connect to the VPN from my iPhone !
I am having a similar issue. I did what you have suggested here by using the older airport utility and haven't had a chance to test it fully yet.
Did this cause any issues with the icloud after you opened these ports? I can only assume that they put the warnings to not use the ports for something else for a reason.... and its not worth it to me if it screws up my icloud.