for static ips you can use the ipindex of a friend of mine - in my opinion it´s the ip index with the latest datas.
ipindex.homelinux.net
but it dont see an real possibility to track down changing ips and coming through firewalls of huge companies (that´s nearly bullshit) - they get the external ip and that´s it and mapping it is nearly impossible, if you got some dyndns host in the company you could do it - but i bet the company wouldn´t be too amused.
hope I could clear some things... anyway if you still consider testing it - I´m really excited if it will work (but I´m more than sure it will not!)