Extend visa on the day of arrival

Hi,
I want to extend my entry permit in Xiaguan after a night on the train. And leave the same day to Kunming. In this case, I won't be able to provide any registration from an hotel.
The problem is that my entry permit expires the day after I will ask for the extension and this day is a Saturday (PSB closed). I have no choice but to apply on the day of my arrival in Xiaguan...
What can I do? If I go to the Police and show my round trip bus tickets, will they be able to register me so I can apply for an extension?

Thank you,
Serima