After being locked out of iMessage, I got it working again by re-using the ROM and MLB values from my older 'Lion' MacBook Air. All it took was setting the values in Clover RtVariables and calling Apple support to unlock. I didn't change any other values like smbios.
If you happened to have an older original Mac, here's how to reuse the ROM/MLB values:
From terminal, type the following:
nvram 4D1EDE05-38C7-4A6A-9CC6-4BCCA8B38C14:ROM
nvram 4D1EDE05-38C7-4A6A-9CC6-4BCCA8B38C14:MLB
The MLB value can be used 'as-is' in RtVariables.
The ROM value needs to be hex coded. For example, a code of "%03%1e[%8foH" would be "031e5b8f6f48"
I used this online string to hex decoder:
http://www.string-functions.com/string-hex.aspx
1) copy/paste the ROM -> %03%1e[%8foH
2) replace each sequence of %xx with a # -> ##[#oH
3) convert to hex -> 23235b236f48
4) replace the "23" sequences with the original "%xx" codes -> 031e5b8f6f48
Put both values as STRING (!) in the clover config.plist:
<key>RtVariables</key>
<dict>
<key>MLB</key>
<string>xxxxxxxxxxxxx</string>
<key>ROM</key>
<string>yyyyyyyyyyyy</string>
</dict>
Reboot and make the call to Apple support to unlock.
Note: as outlined multiple times by jaymonkey, cloning mlb/rom values might cause Apple to blacklist the values so don't do this unless you 1) own the mac (!!!) and 2) are willing to take the risk of blacklisting. Since my old MacBook Air does not support iMessage, cloning seems save and, for now, a working solution.