I ended up desoldering the ROM from the memory card and putting on 0.1" headers. After a healthy bit of debugging with an oscilloscope I read out the contents and for the first time, it looks promising!

This is what should be on the memory card:

ROM dump:

Zooming in a bit:

That's looks like the fireworks and cowboy on there! I'm treating every 2 bytes as an x and y delta coordinate - so the reason for the long trail in the initial image, is there's headers or control bytes or thread specs that I'm treating as a stitch causing it to trail off. Next steps will be identifying those areas and parsing them