[ghoul2]

That needs the "boot host" rpi to be a usb device or usb otg - afaik, only the pi0 and pi4 have usb otg. These can emulate a usb storage device via the usb gadget subsystem. Additionally, only RPI4 has USB boot capability, and it does not work with all usb devices.

More importantly - something like this can only be hooked up to _one_ boot device, so the usb hub and multiple pi's are a no go. I don't see any advantage compared to just using a usb stick instead of making a rp0/rpi4 pretend to be a usb stick.

You should look into network booting the multiple pi's - if thats suitable for your use case. You will still need an sdcard in each pi to provide the network bootloader, but once the boot is done, the sdcard isn 't used anymore (until the next boot)