How about you do as on the link? Since it seems you need to pre-register anyway:

"To inquire about transit hotels, please check with Air China sales hotline or Air China's representative office nearest to you. "
" If you are on a night flight and you have reserved Air China'€™s transit hotel in Beijing, you are requested to head to Air China'€™s transit service counter facing Exit B on Floor 2 of Terminal 3. "

(Ok, maybe I'm spoiled here, since I've a Air China city office right next door where I work.. but it wont hurt to give them a call if they're not convenient located)

On the other hand, if all I wanted was some sleep, the Ibis Airport (including shuttle service and "breakfast") usually was available for less than 200 RMB..