My friends who actually managed to get on board an LH bird with Connexxion equipment used
Skype very successfully.
It is equaly bad as through a lousy firewall or proxy but it does work.

I think that it is entirely futille to rely on passengers' well behaviour these days. Manners are
not in fashion, and free phone calls attract weirdoes much like the light does moths.

Blocking ports is not going to work as there are too many serives using very basic protocols.
But introducing a large amount of jitter on the IP packets would not inhibit all other services
at all but make gaming and Skyping nearly impossible.
Which again is a great benefit for all customers on the plane as it assures low noise levels
and still allows you to fire urgent messages.