Using Nagios to monitor multiple sites

I have multiple sites that I need to monitor using nagios. Unfortunately there is no static VPN connection so just using standard ip addresses or computer names will just not work. The solution will have to work through NAT and hopefully involve little firewall alterations.

I have done some research on this and I happened upon Nagprobe. I was wondering if anyone else has come up with a good way to monitor computers through NAT, and what I could do to set up my own Nagios system. I did want everything to end up coming back to a main server/website at the main office location.

My current idea is to set up my main server and then NRPE or NSCA the information back from another Nagios server that is set up at that site. The computers are pretty much all Windows so I was thinking of using vmware to virtualize the nagios server that would send the information back.

Any ideas and/or what other people have done in this instance would be very helpful.

Thanks in advance.