Thanks to Richard for sending in this video from Chris Burton on YouTube.
Raspberry Pi 2 + Cluster Hat + 4 Raspberry Pi Zeros
Pi Zeros communicate with the Pi 2 (or A+/B+) as USB Gadgets (g_cdc for network and serial console) via a USB hub on the hat which connects back to the Raspberry Pi 2. Power to the individual Zeros is controlled via GPIO pins so you can easily power cycle a crashed Zero.
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