That's actually very good Steve. If I were to redesign the KIWI-OSD, it would have a video splitter too (and indeed I use one in my obervatory).
The Kiwi makes use of an external GPS to get a 1 Pulse per Second signal from a number of sattelites which it does some clever maths to in order to get a milisecond accurate timestamp. After time aquisition, The time is read off the device i think.
If you are ever up for a project, you are well on the way to filling nasty hole
(there are others on the market, but way to expensive!)