Interesting idea!
Long term, There are some really interesting places I want to go with having hierarchical/nested render extents. Mostly this is for data propagation from large to small scale – some planned future devices work more on a continental scale, so you want to be able to use them to define very large scale elements and then have that data available inside your highly detailed, small render extent.
In the more near term, I think we can get most of the way to your request with a planned improvement to the Checkpoint device, detailed here:
https://forum.world-machine.com/t/cache-terrain-device/5310/2
You could basically checkpoint-capture each individual high res area, and have them feed their results into a more global scope. The only additional device required I believe be a “Select render extent” device that outputs a mask indicating which areas are inside of a given render extent, which is a trivial addition. You could then merge the output of the detailed areas from captured checkpoints with global data calculated from the largest render extent.