Do you have Wechat "wallet" in RMB/CNY currency?
Does your BOCHK account have HK RMB currency that can be tied/linked to China RMB account ?
Does it sound complicated? Yes, because it is. Bank of China Hong Kong can create a new RMB account for you that can be linked to the Wechat RMB wallet. but.... inorder to do this, they want your "Home Return Permit" ( 回鄉正 )
Since you are not in HK, is there anyone in HK that can get these documents to the bank for you ? If you donot have a "Home Return Permit" that is already registered with the BOCHK, then you are most likely stuck.
If you do not have "Home Return Permit" ; then this would not work.