Yep, now I remember it, and if my memory is correct this time, there's WorldBuilderRandomItems.lua which is called when the WB random map resource placement is checked, and I've hooked the map generation of YnAEMP in the goody hut function of MapGenerator.lua, because I hadn't found any other place to do it. (and so I need to check the random placement for goody hut for all the maps I use in YnAEMP)
So I believe WorldBuilderMapLoader is not available as a Lua file, it must be a function or file in the c++ code.