So, what you're saying it that if someone had a character that was headed to one of the maps that you're missing, there's information that is created while in the loading screen? Or?
And yeah, Ascension Island and Sky Rim are a plus, if you can get ahold of them too. I believe Ascension has the same height map as Bonfire but from what it sounds like, is that you need more data of the map.
Sorry, I'm terrible at explaining things. XD
I'm pretty sure I've already broken some rule by even making this map pack, so I'll try to keep the process to how I get heightmaps simple.
Basically, as soon as you go into a portal the map is loaded while at the same time (or maybe after) your game tries to tell the game server you've entered said map. However, since the admins have blocked people from going to the old 1.13 maps, after you go through the portal your game will time out and you get an error message saying you've lost connection to the server.
But if you don't click on the reconnect or quit options the map and its information is still loaded in memory and you can extract it with the right tools. For example, here I've gone from lonely cave to Ascension Island and even though I'm not actually on the map I can still browse its contents.
However, unlike community made maps the ones that shipped with 1.13 have some kind of block that prevents you from creating a map in the editor and pointing its destination to one of the maps that aren't connected to Lonely Cave.
So, unless someone knows how to bypass this and make the game load the 1.13 maps through one created in the editor, I don't think it's possible to get the remaining maps unless the admins remove the restriction.