Defining connections via JSON?

[NetSysFire]

Currently roads, subways and sewers are C++. In OVERMAP.md it even says Specify a C++ mapgen function for a LINEAR feature variation. Prefer JSON instead..

mapgen_(curved|tee|straight|four_way|end) seems mandatory for such connections but it requires C++ mapgen functions. So is there really no way to define connections in JSON? Looking into the code (which I did in #52807), there is clearly a normal object you could most definitely replicate via JSON.
There is no mention of it in OVERMAP.md, but can the method be something else than builtin so you can smuggle in JSON?