I spent way too long to get it working and I was almost ready to give up.

I tried the standalone ISO updater with a LOGO.GIF file. It recognized the image, but after reboot the original Lenovo logo was still there. No error message, no warning, nothing.

Same with the Windows firmware updater.

The one thing that finally worked was a Winflash64.exe incantation found buried somewhere deep in the Lenovo forums 😠

the incantation 
Winflash64.exe -patch -logo logo.gif -exit

Posting for information only. This worked for me, but may - and probably will - brick your machine. You have been warned.