As you have realized, in this situation both sides of the VPN need to do NAT to overcome the routing issues for the communications subnet.
These are the ScreenOS side instructions. I am not sure, but think all these features are available in ScreenOS 5 but the web UI is completely different.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB5346&smlogin=true&actp=search