As a data point...

I wanted to do the same thing (apply using somebody else's offer code) but I wasn't able to because I was unable to edit the name field.

However, today I just kept visiting the Amex platinum card site in incognito mode, and about the 4th time I visited it, the 100k offer populated!

I've confirmed with multiple reps that I will in fact receive the 100k offer. Of course, I guess nothing is concrete until I have it in my hand...

Edit: This offer is for the personal platinum card.