You can use a chip clip, but presumably you rejected that idea because your objective is for this to be replicable by the kind of people who don't know what a chip clip is.

I knew I wanted one, but I didn't know that it was called a "chip clip" and indeed "IC clip" and "IC test clip" seem to be a better match.

The clip will give you a electrical connection to the SPI flash but I'm not sure you'll be able to talk to it without jumpers on the board. Is it possible without jumpers?

Normally.

I'd be very surprised if there isn't a filesystem driver for SPI flash. Linux obviously can speak SPI and SPI flash is extremely common in a lot of applications.

I mean once assembled. In my design you can remove card, put files in, boot again, use the files.

Theoretically you can use one of these: https://www.tindie.com/products/bobricius/micro-sd-card-to-s...

I haven't tried it so I don't know how well it works

It looks like a passive mechanical adapter. I'd argue that a SOIC-8 programming clip thing would be better. This looks like you need to write a decent amount of code for a computer to talk to it as easily as a FAT-16 SD card.

You're right; it looks like SD cards implement a protocol on top of SPI. They don't just map the content to SPI addresses. So a flash chip won't look like an SD card to the OS

Right. Unsure if it's possible that any SD controller has a secret SPI mode. This seems like a ridiculously niche product. Which is fine but just something to keep in the toolbelt.