Add new "network channel about to connect" event to nsIObserverService?

In

    https://bugzilla.mozilla.org/show_bug.cgi?id=780087

we'll be blocking B2G app's traffic if they exceed a quota, and/or only 
allow it if the phone is in a wifi network, etc.

Honza came up with the idea of adding a CSP policy to do this 
monitoring, but sicking comments on IRC that "putting it in the CSP code 
sounds like a scary hack though given that it's not CSP code at all".  
So perhaps CSP is a bad place for it.

We could use http-on-modify-request, but that only fires for HTTP requests.

I'm currently thinking we might want to add a new nsIObserverService 
notification that's issued for "necko channel for app AppID about to 
connect on network interface FOO".   How does that sound?  Am I missing 
some other clever way of doing this?

Jason