Hainan student baggage allowance

Hi, I'm an international student. With Hainan you're allowed 40kg as intl student, but you have to get an "OK stamp" after booking your ticket online.
Is it possible to pack 40kg and get this OK just a few minutes PRIOR to departure??

(It says the following on the Hainan page: "[go to]... Hainan Airlines Airport Desk with your valid Visa documentation, valid Passport, Student ID or Letter of Admission and photocopies of this documentation in order to activate your Special Baggage Allowance") .

Is there a Hainan Airlines Airport Desk at Nanjing Airport?
Thanks!