Now that's an interesting idea, RAID across multiple separate networked devices. That'd be an interesting protocol to build! (I know you could do it with iSCSI across multiple devices, but I'm imagining a more native protocol that handles autodiscovery of the volumes across the network... you mount your "Resilient Storage" volume and all the RAID is handled for you.
I still have two Time Machines with working drives, honestly it's too bad there isn't a Linux firmware for these things because they're actually decent little machines, if I could flash one with Linux I'd probably drop in an SSD and carry it around as a travel router. Yes, I know there's lots of other options for Linux based portable routers, but we can't argue with the design...