Got by with a Rubber Mallet.
Just last night, I messed around with the washers, nuts, and threaded bar for about 30 minutes. Didn't get anywhere. Rod kept creeping and scratching the inside and top of the headset.
Grabbed the rubber mallet.
Just 4-5 whacks and it was done. The sound changed in pitch when the cups were seated.
Your results may vary. I can't recommend it - I can just give witness that it worked.
EDIT: It's worth noting that I wasn't doing this on a chi-chi carbon frame with a Chris King headset.