Thanks for the link with the history of changes.
I'm not sure if side-chain multisignature has a role in block production in this scenario.
Suppose that the actions of @steemsc are made by consensus between several parties, like the witnesses of the side chain. In this scenario, @steemsc is a multisignature account, and one action requires 17 of 21 signatures to be performed.
The authorities (active/owner) of @steemsc are updated in the main chain as the witnesses change in the side chain.
I think that it is what Dan is saying in his article. In fact, he is saying that this is the way Lisk is working, and that we could do the same in steem:
Lisk relies on 3rd party multi signature solutions to evaluate the code and authorize any transfers out of the smart contract.
...
App developers could use Steem and BitShares to implement smart contracts via Side Chains in the same manner