[Steve in Pudsey]

I think the question at present for that scenario is where the sting would come from, neither the News at Six or the World News programme would be able to do anything without interrupting their ongoing output so it would require a third gallery to pass through, as the News Channel operates by switching between galleries - it doesn't have it's own presentation suite to play anything else in.

This type of thing, at least, will be solved by the upcoming changes.

Actually, I think the current infrastructure allows a sting to be played without a third studio.

A video I captured of WN opting out of Sunday with LK recently showed that Studio B has the capability of fading out the audio AND inserting a sting into one of the TX line out to WN Pres WHILE the main programme continues on the BBC One/NC TX line...

youtu.be 

I'm not so sure.

Studio C picks up Studio B output as an outside source. Plays sting and weather to World News.
World pres pick up off the back of the weather with trails and World countdown
Studio C cuts back to Studio B and switches News Channel to C output
Studio C plays the "continues on BBC One" announcement and the News Channel countdown


Although I'm sure the same technique that was used to do the 8pm summary playing weather to network/HD and the London opt from the same studio could be used as a pre-planned thing.