I'm not sure of what mechanisms are currently in place for updating nodes, but that same mechanism would still be used. (ie. on the live site).

The difference would be that non-god authorised users updates would be made into the look-aside table rather than the real database. There would also be a list of prohibited nodes. Attempts by non-god authorised users to read prohibited nodes fails. Updates to a node can only be made if the user has previously read the node.

All of the authorisation and node update nodes are on the prohibited list and can only be run from the live system. The LAT is only used from the test system, so no updates can be made there.

That's not very well described, is probably flawed, but it's almost certainly a waste of effort to even think about it further.