1.) LogMeIn has a working client for Raspberry Pi:
http://raspberrypi.stackexchange.com/q/10110/17815
This solves your problem if you're having enough budget.
2.) I'm having the same problem (and low budget) and I think about the following:
- install VNC on the RPI with autostart, so it connects to my pc without interaction
- let RPI check every 30 seconds the URL
http://example.org/PleaseCallMe.txt
- if the URL is available RPI starts VNC (and connects to my PC)
What I need to know:
a) is it possible to setup VNC with an autostart connection?
b) is it possible to let VNC run only on port 80 and/or 443?
c) how to get/install such a script to check URL and start VNC?
EDIT: Hmm.. maybe it would be better to start an OpenVPN tunnel, instead of starting VNC. And kill the tunnel after the URL is not available anymore. I think this would be much more flexible and works with Port 443 only. What do you think?