I'm Nigerian working in China. i need to get an S1 Visa for my wife and daughter. Kindly advise!

Hi,
I’m Francis, a Nigerian citizen presently working in China (with a valid Z type, Work Permit, Residence Permit and Employment Certificate) on a 6 weeks on, 3 weeks off rotation schedule. I’ld like to bring my wife and little daughter on my next trip late March, 2015.
We’ve been trying to get a Dependency type for my family with all the required documents provided. But Chinese Embassy in my Country Nigeria are refusing to attend to us. They request for LOI from the chinese govenment even though I’ve presented the one with which I got my Z type just 3 months ago. They even refuse to grant a Tourist L type saying they only grant Tourist type to Nigerians with Dual citizenship of another approved country.
I contacted Bureau of … in China and was told they only issue LOI for Business and Work Entry Permits.
My Question: What official Letter or proof can I get from the Chinese Government here in China to show my eligibility to the Chinese Embassy in Nigeria (I understand many Nigerians have painted us black here in years past, but I’m not one of them and I have all valid Chinese legal documents to proof)? What department can I contact or how do I get this document? I’ve sent 2 mails differently to the contact email on the chinese website with no response, I’ve called all the lines but not valid, the only one that went through was picked by a lady who speaks only Chinese. Kindly provide as much information as possible.
Thanks and sorry for the lengthy comment.