SCO would be a very good banner, but that would be huge. Essentially SCO will become effectively and officially a new "Warsaw Pact". So far China, at least, is avoiding the equalization of SCO to a military pact. This is another hindrance to the SCO banner at this point.My reply to both you and Siege is that if China were to join the fight , it would no longer be a Russian led operation but an SCO led operation.
It would be a very convenient banner for other countries to rally too, perhaps easier for some than simply the Russian banner.
The other side of the coin is that it would be a very high stakes situation for an initial outing and there is always the danger of too many Cooks.
My guess is that Moscow would love the extra diplomatic cover but fear possible negative operational effectiveness. It would boil down to the precise calculation as to which of the two factors are regarded as the most important.
Besides, even if China is joining, other central Asian SCO members may not be willing to. The new member Pakistan will surely be in an embarrassing position for many reasons.
Even if only China and Russia provide material action in Syria, while other SCO member only provide the SCO banner, it is still a too big political statement for the other members.
To sum up, SCO is far from ready to conduct a joint action even only as a banner at this point of time.