There are few rumors and facts about China's oversea involvements in the past speak the contrary.With the last point I agree wholly. But I reason that Russia might be damaging, to some extent, its reasonably good relations with Saudi Arabia. It might be prudent for China not to do the same. Of course Saudi politics are difficult to judge especially for total outsiders like myself.
And perhaps the Saudies are now mostly concerned with the terrible accident during the hadj.
1. China sold weapons to both Iran and Iraq during the Iran-Iraqi war without damaging long-term relationships to either countries. I don't know how China managed it, but she did.
2. China has sold ballistic missiles to Saudi Arabia without damage Sino-Iranian or Sino-Israeli relationship.
If behind the door SA complains, I am sure China would have told Saudis about Uyghur fighters in ISIS. Everyone including SA knows that nobody would let a threat to one's interest runs free.
Regarding the possible Chinese involvement, I am sure China will take a very low key approach, concealing all official connections such as no Chinese marking on assets, no Chinese personnel outside the Syrian military compound, no Chinese military uniform etc.