During on one of our test, we are require to extract Product Key from Surface RT. We found a solution to do that.
Credit to Myriachan (http://forum.xda-developers.com/showthread.php?t=2442791) . Save below script to batch file (*.bat)
::' Windows RT 8.0 Product Key Dumper by Myria of xda-developers.com
\CurrentVersion" /v DigitalProductId | cscript //nologo //e:vbscript "%~f0")
Run above batch file on command prompt to get the product key.
We have tested it and it work on Windows 8.1 RT.