Since I started this thread, I'll report back what I've found installing 3 BSD's on a T420. All three work well with some minor differences here and there. OpenBSD is what I'm currently using on it, but that will change many times over since it's not my main machine and I use it to try out other operating systems.
FreeBSD: Everything worked, but required installing/enabling certain packages/services related to intel/thinkapd to make some things work. Bootup was sort of slow (extremely unofficial benchmark). Feels like Free takes a little bit more to become a desktop OS, but I would use it again on another machine.
NetBSD: Couldn't get wifi to work via wpa_supplicant, but I still have no idea why. Plenty of other people seem to be able without issue. Other than that, no problems. Lots of respect for all the work NetBSD developers do.
OpenBSD: after fw_update, wifi works with a simple config in /etc/hostname.iwn0. Everything on on the T420 (trackpad, the volume/mute/mic keys above the keyboard) work out of the box with no config on my part. Suspend/resume works out of the box. Comes with CWM installed so that's a bonus lol. Do have to change some settings in login.conf for desktop usage. Fastest installer for whatever that's worth.