I'm pretty sure that the Remote Assistance feature in Windows will allow an arbitrary machine behind a NAT firewall to be connected to from the outside. Like in the Mac solutions you mentioned, however, I don't know specifically how to initiate the connection.

Both Windows and Mac Remote Desktop will only hit a single machine behind a NAT firewall when connecting from the WAN side by default. Not sure if there is any way around that. This isn't an issue if all the machines that need to be connected to are on their own LANs with their own WAN connections (different homes for instance).