This is probably a stupid idea, and i know next to nothing about the interfaces, but could we not abuse the CSI/DSI ports to connect Pis together?
From what little i've gathered these are basically unidirectional, point to point, high speed, serial buses for cameras/displays. I'm not sure if this is even possible, since w'ed have to get quite low level access to the hardware (bypassing camera/display drivers or whatever).
Another option might be abusing hdmi, but with hdcp and all that nonsense that might be even harder.
The last resort would probably be to hijack USB.
Seriously, all of these would require some heavy lifting software-wise, not to mention some very precise interconnects (those squiggly traces on high speed buses are there for good reason). Quite likely it would also require custom switching hardware if you'd like to have more than two nodes in your cluster. By then costs are rising quickly...