With a little effort, its not that hard to make a video card. Honestly. Sure it takes more dedicated time and resources than 99.9% (including myself) want to put into this hobby, but it can be done.
video cards of this era are just framebuffers with a DeclRom that allows a driver to load in the appropriate location to allow the card to work, since its an internal grayscale card that doesnt use an external monitor, it would take entirely over the internal video hardware. In those days, all the drawing and rendering, instructions, etc were all done on the CPU and just sent to the frame buffer. Either in internal RAM, or a DeclROM telling it otherwise. The video hardware itself was nothing more than a DAC with dedicated frame buffer memory.
the DeclROM has to contain at the very minimum a low level driver that sets up the screen buffer and hooks the OS to use it, otherwise the screen would stay black and nothing would happen until a software driver loaded.
They make dedicated embedded GPUs these days, so it would even be easier to make it work and being able to support more features than originally by moving all the drawing and rendering from the CPU over to the GPU effectively making it an "Accelerator". but again. Driver....
I think there is a designing hardware, or an inside macintosh book that has an example of a video card declROM and how it works.