But thinking outside the box, it’s absolutely possible: if I wanted to, I could create some base classes in lua (getting/setting note data and so on), and connect them to some external program which would display the piano roll itself. So, building e.g. an HTML5 application with modern frameworks which would receive it’s data from a pure lua “backend” in Renoise.
As a very very very silly old example in Linux of something along those lines → https://forum.renoise.com/t/experimental-miniroll/40559