Well - I gave this a go and it does seem to work extremely well. Both the in game chat lobby and game connections came up and I was able to enter a multiplayer game across the Internet for the first time in a very long time indeed.
My criticism is that there seem to be a lot of lag issues. The bandwidth across the virtual network is very poor indeed at times. I imagine this is because all traffic is routed through a central Tunngle hub which is struggling with the load. I did notice that whilst connected up, all other Internet traffic went at snail's pace.
When I was hosting, Batgirl was clicking and waiting some seconds for movement, the other way around I had 0-3 seconds of wait.
We'll try again another night and see how it gets along.
Might inspire me to give Hamachi another try, though I've never had any luck with it at all.