I'd like to set up an environment where I can use VNC to remotely support my clients over the internet. No VPNs involved. I've used the UltraVNC repeater in the past, but the problem is that it requires a dedicated Windows server. What I'd like to do is as follows:

VNC Client (me) -> NAT -> Internet -> NAT -> VNC Server (the person I'm offering support to)

I'd basically like the same functionality that the UltraVNC repeater offers, but the only internet environment I have to host something on is a Linux shared server (standard hosting - PHP, Apache, etc.).

Requirements:

1. Multiple platform support for both Client and Server - specifically Mac and Windows
2. Allows for connection with multiple NATs involved (Client and Server side)
3. Will allow me to use my existing hosting environment for any repeater that might be involved

I believe the way this would work is that the Server (the person I'm offering support to) when online would connect to a listener on the internet. When they needed support I would connect my Client to the same listener, see them connected, and use the listener (man-in-the-middle) to piggyback my Client to connect to their Server.

I'm open to using any software (not limiting myself to VNC) but would prefer a FOSS solution (which is why I'm leaning towards VNC). Any advice would be greatly appreciated.