Received the 620.
The trick from Trying to install OpenWrt on a Velocloud 520-AC - #7 by frollic worked here too, no need to reflash the device.
The instruction on how to reflash it using DiagOS doesn't work for me, DiagOS fails to install onto the built in eMMC and SSD, I get the same error @dmsmartins had.
DiagOS installer have the watchdog auto reboot problem too, but it can be disabled once you end up at the prompt, by running i2cset -y 1 0x22 0 0 b.
The i2cset command doesn't however work in OpenWRT, I get a write failed
root@OpenWrt:/# i2cdetect -l
i2c-0 i2c igb BB I2C adapter
i2c-1 i2c igb BB I2C adapter
i2c-2 i2c igb BB I2C adapter
i2c-3 i2c igb BB I2C adapter
i2c-4 smbus SMBus I801 adapter at e000 SMBus adapter
root@OpenWrt:/# i2cset -y 4 0x22 0 0 b
Error: Write failed
root@OpenWrt:/# i2cdetect -y 4
0 1 2 3 4 5 6 7 8 9 a b c d e f
00: 08 -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- 44 -- -- -- -- -- -- -- -- -- -- --
50: 50 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --
I've already managed to install OpenWRT onto the built in 16GB eMMC.