In theory, it's actually possible. Ren'py is based on a version of PyGame, and includes a lot of it. You'd build this as a "Creator Defined Displayable"
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which gives you pretty much full control (in Python) on what is displayed on the screen, plus access to raw event stream.
Of course, it would take a lot of Python programming, and whether the Ren'py version of PyGame contains everything you'd need is an open question (PyTom build a custom version built on SDL2, and there are a few things he consciously omitted), but if you're willing to go to that level, you do have a lot of power.
Not saying that something like this would be easy, or that Unity might not be a better approach, but there _is_ a lot of power lurking there.