How to create SAN with OpenWrt?

I have about 20-30 Linksys EA6350 around. This router hardware is quite powerful, with USB 3.0 support. I also have hundreds of spare HDDs. What comes to my mind is using them to create a Storage Area Network. Each router shall use USB hub and LVM to combine 3-4 HDDs into a single pool of storage, then share that pool as a single device for a centralized server, which runs MD RAID across them. Sounds like a fun project to do in my spared time.

But how can I do this? OpenWrt's tutorial only covers NAS (filesystem level access) but not SAN (block level access).