no cracktro would use that much RAM! Wouldn't even run on stock original A500...
blitter fill mode isn't that expensive, I reckon you could draw the outlines with the CPU just by storing tables of width as a function of Y (for every different size if you like). XOR them into the bitplane and fill them all in one go. I never see two of them overlap in the video, or maybe I'm wrong. Then plot the stars on top. We assume the stars are behind, but it makes no difference if you draw them last.