I came across a mention of this the other day: SDLPT (https://vintage2000.org/sdlpt), a parallel port adaptor for an SD card. Has anyone tried one? I think they might be particularly useful on a PPC512 or 640, where it's not straightforward to fit a hard drive.
Looking at the driver source, it might even be possible to create a .FID driver to make one work on a PCW9512 / 9512+. (I don't think a CPS8256 or other parallel port lets you read the SELECT line, which is necessary for card -> host communication, so it would only work with the 9512's built-in port).