I actually quite like this idea
If they marketed an ARM based add on like this that just gave out ISCSI or AoE, it would be pretty cool. I could scale out my storage server just by plugging in more drives into a switch. Would not need to worry if I have enough SATA ports, or if I need another SATA card, or even need to have the drives in the same place as the NAS.
Both ISCSI and AoE allow for concurrent access, so could even distribute storage between my machines as and when needed, without having to unplug and replug drives.