I think it is necessary for useful prototyping to allow the user to create the UI elements with the script, because then the prototype would be exactly like the final module, when implemented later in C++. I understand that it would be a lot of work to implement and maintain in the VCV Rack code itself, so I guess it is up to module developers like me to do it in a module 
I just did a quick hack to use LuaJIT to create inputs and outputs, and to process audio:
The compiled plugin is 216 kB (stripped). I used LuaJIT as a shared library. Might be better to link it as a static library, but I tried it and there was some error messages with fPIC etc., at which point I gave up to try this. But with the shared library it works. Link to the source code, including the echo script example, and compile instructions are in the video description. Tested for Linux only at the moment, but it should work on the other platforms as well, I didn’t use anything Linux special.
Would be nice to create the inputs and outputs without restarting Rack, but I’m not sure if this is possible. For example the config-call in the constructor of the module is supposed to be called only once. My hack to workaround this is to use a fixed max number of inputs and outputs, but would be better if this would be dynamic and controlled by the script.