I don't see why not, software renderers exist, just point it at a "surface" made from "pixels" which are actually block characters of different colors.

>Can OpenGL work in Notcurses?

Yes, this is quite easy to do with OSMesa, or with EGL_KHR_surfaceless_context. With the latter it will be hardware accelerated too.

One thing I once did was capture the screen and stream it. On the other side I watched it using mpv with libcaca as the output.

Graphical desktop on the terminal.