Package: Schematic Editor
I wouldn't mind being able to manually edit .mts files, but at least I can create them now. I will definitely be using this newfound power, but for what is still to be decided.
The Lua2Mts mod (https://content.minetest.net/packages/Neuromancer/lua2mts/) allows you to manually modify schematic files in lua and then convert them back to mts. So you basically get what you are looking for here. It fills the gap in the Schematic Editor mod.