Hi there,
I agree, this is a very useful idea!
It has a lot of applications for tiled builds, working with water, etc. And funny enough I’m right on the same page with you - this almost made it into build 3028! There are some corner cases where it wasn’t clear how best to handle, and also the question of where the data should be stored - putting sometimes-gigabytes into the TMD is a bad idea, so it should instead go into a companion .state file or something similar, which then requires some additional plumbing throughout WM.
This was actually going to be added to the Checkpoint device, transforming it from an inert routing aid to a useful utility device. An initial proof of concept of this exists but the corner cases held it back. I do hope to get it out into one of the next couple builds!