I got around to testing this out this afternoon, and it works perfectly for remembering the d-map to return Link to, but there is one small hiccup, but I could have worded my OP (even though it's not really the OP) better . As seen in the short video below, the d-map remembering works fine, but I should have mentioned that not only are these going to be from different d-maps, but also different screens (and maps in a real quest if that matters, this is strictly a test).
The exact purpose is essentially a way to have parts of a world change drastically (using different maps, d-maps, palettes, and music) but still be able to enter caves and whatnot in that same area, and only be able to get an item once. Inversely, with this method, I can also have areas that do change internally between visits by not using the script. That way, the player can't miss stuff if they didn't get it in a certain period, and I don't have to make a crazy amount of duplicate interiors with elaborate screen warping and secret carryover triggering to get the results.
It's essentially a more hacked together "events" system than using dummy items.
In the example, the screen Link is returning from is set to the position of the screen at the start (Screen 03) (Spring) from the interior d-map. Side warps take you to different d-maps on different screens, and when exiting the cave from one of those, it does remember the d-map, but there is no way to specify alternate warp coordinates if that d-map is a variant of the original screen, and uses a different map/ screen. Again, shoulda been more clear in my OP.
please excuse the horrendous test screen design