All of the English regions' baloons era clocks were on tape - the last minute leading up to 1300, 1800 and 2200 were delivered to each centre on a tape. This was the first time in years that the regions were asked to do clocks into the "News Hour". The regions hadn't introduced the national news over a clock since the eraly 80s I think - and back then they all had mechanical clocks - as far as I know they were never sent GNAT devices (Generator of Network Analogue Time !) to go with their COWs (Computer Originated World) So for the launch of the newshours with integrated heads and halfways, and the requirement for regions to have a clock introducing the national and regional presenters, sending a prerecorded clock was seen as the most cost effective way of doing it. I remember a mate of mine dubbing them all off and grumbling about how dull the job was. I also remember watching the first regional clock go out at lunchtime ... from a certain room on the 4th floor of Television Centre if I remember rightly...
Anyway, should a regional clock like that fine mock ever return, (and I don't think it will) it's highly likely that it would again be delivered on tape and therefore any keying and transitions etc would be simply part of the edited video delivered.
I remember being told that Pebble Mill had a GNAT, or I read it that way. It could just have been a 12 hour VT
of a clock. It was to be used when Net was switched up to Brum as part of DR. I'll have to try and find where I read that, it might have been on a pres site that has long since closed.