If you are behind a router (without port forwarding) you will not be able to accept any incoming connections to your gnutella server port at all, so this is no issue in this case. You should still be able to connect to gnutella however.

The simplest solution is to add proxy support to LimeWire and set up a proxy on the router, - I've got some SOCKS code from an open-source java httpclient on my harddisk, I meant to make it usable for LimeWire, but I did not have time since I had to spend most of my energy on my education (at the university). Since the semester has ended I think I could add it pretty soon and submit the patch to the LimeWire team.

Setting up LimeWire itself to relay connections (including uploads and downloads) for other clients is a nice idea but I think it's not absolutely necessary for that purpose. There are SOCKS servers in java that could do the relaying stuff on any plattform.