For geek cred you could put in another Network or Storage adapter and do ISCSI or FC. SAS would allow 2 to share storage, however I don't think SAS supports more then 2 initiators. Directly connecting every host to a pool of Flash chips would likely require a custom controller for the flash and a way to present it to each Raspberry Pi

This will almost certainly not make sense economically, but it seems a lot around RPi clustering doesn't make sense from a performance/$ perspective.