Parfocality through firmware
I just tried that with my Nikon D7100 plus Nikon 18-140 zoom lens. The lens is most definitely not parfocal and I could not find a focus mode that seemed to try to do what you suggested. (first I focus on something, then I change the zoom and hope focus does not change... it changed a lot with the 18-140... it takes a shutter half press to refocus) I am not running the very latest firmware however.
I just tried this with my Sony RX10 too (not a DSLR) That camera is targeted for photo-journalists (according to Sony) so the behavior you described would be useful for that group of users. Focus did seem to stay pretty constant with zoom, and then when I stopped zooming, active AF would be hunting in low light. I did not try to figure out exactly what was going on. But it did seem like the RX10 was smart enough to make a guess at where to be starting with the AF once I stopped pressing the electronic zoom lever. The AF hunting only began once I stopped pressing the zoom lever.
Sounds like you are interested in AF systems. There is a handful of people here on dpreview that seem quite knowledgeable about those things. I don't know if they visit this forum though. I see the most informed discussions on the Nikon and Canon forums.
Here is an example article from a Canon regular.
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|Diver's Watch-2757 by vbuhay|
|Cerulean Sukhoi by cjf2|
|Cheetah in the wild by cmgpereira|
|Kilchurn Castle, Scotland by johanmieke|
from Ancient Castles, Forts, and Defensive Structures - EXTERIOR