The Sandy Bridge T520 is now supported by Coreboot after last month there was T530 Coreboot support. The T520 is now supported with Intel Sandy Bridge CPUs and Intel BD82X6X Southbridge.
The Lenovo T520 support landed today with this Git commit. Before trying out this open-source BIOS/UEFI replacement on your ThinkPad T520, be forewarned that suspend-and-resume doesn't work with the current implementation, when going through the shutdown process it doesn't sometimes work as expected, and there's mysterious hard drive I/O happening, among other early issues. More than 2,000 new lines of code were added to Coreboot for supporting this older Lenovo ThinkPad to rid it of its proprietary BIOS.