What is the process for a customer to request additional $0 users?

NJUSTENI believe the customer can't request these free ($0) users anymore. You mentioned that the customer uses the original BttC promotion, which has been deprecated for a while now. 

Requesting additional free $0 licenses must be done within 60 days of original purchase date.

As the customer purchased the promotion more than 60 days ago (promo been deprecated longer than that), they are not eligible to request the free $0 licenses anymore. 

You could ask mbsquery for an exception, but I doubt any would be given as the documentation is very clear.. 

The customer can, however, increase the licenses in existing promo subscriptions and thus still utilize a discount for new licenses. 

If the customer didn't already have a Team Member promo subscription active, then the customer will need to pay for full priced licenses in a new NCE subscription. 

Not the answer you're looking for, but I hope it helps you in knowing what you can do.