This is oracle's own solution for proxying: http://oracle.su/docs/11g/network.112/e10836/cman.htm
http://www.kernun.cz has also an sqlnet-proxy. (http://download.kernun.com/doc/handbook/sqlnet-proxy.8.html)
Or you can try to use simple tcp proxies. I used previously stunnel for example. (I don't know how specific was my case, but it worked fine.)