That doesn't work for languages which require all the code at once just for compiling, like C++ (if any of the code involves templates), template Haskell, or Ru

That doesn't work for languages which require all the code at once just for compiling, like C++ (if any of the code involves templates), template Haskell, or Rust (at least, not without heroic efforts). You could make sure the LGPL code only works through a C-API like interface, then you can sub-divide it off more easily, but that can be quite a bit of work.