So yeah, the procedure seems to be still doable.
Looks like I did not want to delete anything I had found, though it is not needed to get the downgrade done.
Follow these simple steps:
1. Grab a USB stick and format it FAT32 with the hidden DOS system files on it.
I think I used HPUSBDisk.exe and the files from usbdos.zip while running some Windows OS.
2. Copy over all needed files.
You will need at least PFLASH.exe, B146.FL1, B146M.FL1 and the TP directory with the winflash.exe.
3. Boot from your USB device into DOS.
4. Type in: PFLASH /sa B146.FL1 (This should downgrade your T420 to BIOS 1.46)
5. Boot into Windows again, open a DOS window and navigate to USB/TP/bios_mod.
6. Type in: winflash /sa B146M.FL1
From the winflash GUI, select "Flash BIOS only" then click on the "Flash" button.
That should be it - reboot and enjoy your modded BIOS.
You may have guessed that M stands for the modded BIOS you finally want.
Besides the whitelist removal, some AMT features are also disabled with the mod.
For the rare possibility that you already ditched all your Windows installer DVDs:
The Internet Archive has one here: https://archive.org/details/HPOperatingSystemDVDWindows7ProfessionalSP164-bit650434-DN32012
No serial needed, but the install is not a "genuine" W7 of course.