This in theory should disable signature verification for the OS partition
Okay, I guess it's impossible to keep this quiet now that people are talking about it.
Yes, changing the product ID in the EEPROM will disable u-boot signature verification for the OS partition:
The boot chain up to and including u-boot must be signed. The u-boot environment is compiled into the signed binary, so there is nothing we can do up to that point.
By digging through their U-boot sources I found references for multiple boards.
Meraki are using the same u-boot binary for both secure (Z3) and non-secure platforms (MR33). It will be trivial for them to close this vulnerability (I won't give them any hints though 
). I think it should always be possible to flash an older u-boot binary to NAND, I don't think they can change the certificate they burned into QFPROM.
I may be wrong though, so if you care about flashing another firmware to your Z3, don't ever let it talk to Meraki again.