By "static", do you mean the IP address of the router?
Jini is built on RMI and RMI has ways around NAT throught specifying "proxy" properties and configuring the router to do port forwarding (or use HTTP tunneling) - but it's probably going to be a lot of work to get everything configured properly.
There's some discussion of the difficulty of this in
this post in the JINI-USERS mailing list.