The block idea sounds a bit convoluted, but I supposed it could be done, thanks for your input. I was naively hoping that more people would use the API to put their modded structures on the map, but obviously it's not going to happen unless this mod gets big. So, do I understand correctly that in this Formivore's mod and many others there is a way to set up any custom schematics to generate naturally as part of terrain? It sounds awfully tedious if you have to manually edit some pre-built schematics to put in the custom block. And having to scan through the whole volume of a chunk is probably going to take too much time.
It is convoluted, but unfortunately that's the only possible way to get it to work. Formivore's can use schematic files exported from MCEdit, and Recurrent Complex (As well as a few other mods I can't recall. Schematica?) allow you to create structures directly in game and have them generate automatically under predefined conditions.
And you could assign the custom block to do something on world tick, like tell your mod there's something here to add to the map of any nearby player and it uses this icon. Scanning every chunk would definitely be prohibitive.
And I still think it'd be a nifty feature for Survival to add a shared marker. Though probably not as elegant as clicking a theoretical 'Share Waypoints' button on the map itself. But an easy button like that seems kinda cheat-y to me for a mod that makes mapping and waypoint markers feel otherwise more survival-y.