How do I recover the MBR on a Thinkpad t420?

The only easy way to do this is with the Lenovo recovery DVDs (and you'd be looking a complete reimage then so it's largely impractical). If you didn't burn these when you got the system you'd be looking at purchasing them from Lenovo, which last I checked was about US$50.

I have no idea how difficult it is to get that partition booting with any OTHER bootloader - there may not be anything particularly special about it in and of itself, but I don't know of any way for you to get it to respond to that Thinkvantage keypress. Fortunately, there's not much of use on that recovery environment that you can't get in other ways anyway.

Lenovo uses a custom MBR so that the thinkvantage button works properly, as Shinrai stated one way to get it working again is a reimage of the hard drive using the Lenovo recovery discs.

Some brands like HP, you can mark the recovery partition as "active" and reboot, this will allow it to boot directly into the recovery partition and load recovery, this is the manual way to recover the notebook when the mbr has been replaced with a stock windows mbr. Not sure if this works on a Lenovo with a grub boot loader installed.

The recovery partition is bootable, you just need to figure out how to boot from that partition to run recovery if you don't have the recovery discs.

Most people make the recovery discs, then delete the recovery partition.