The sprites would have to be hardcoded, instead of being interpreted from images. You would also have to be extra smart about what you're calculating/rendering, but keep in mind that your screen is probably only 40 characters wide, so the amount to render is exponentially smaller.
You'd be amazed what can be done on those older computers when the developer understands how the processors work on the ground level.
At the very least it'd be an impressive tech demo for something like the Commander X16[0], a new 6502 based computer running at 8mhz.