Is your requirement that the "central bank" learns about all transactions eventually, or controls all transactions in real time?
A notary is intended to be a distributed thing. A single organisation notary is technically possible and we provide an implementation of that for testing, but at that point, you are getting close to the point where you could just have the CB host a regular database as done today (unless you wish to combine this app with other kinds of transaction/smart contract that are more decentralised).
I think it's important to keep in mind with any blockchain inspired distributed platform that these tools are only helpful if you're willing to distribute social power, at least to some extent. If your requirement is "entity X sees all data and controls all changes" then a distributed ledger is the wrong tool to use, almost by definition, because you have a hard requirement that the system not be decentralised.
If your requirement is simply to CC a repository on all transactions, and you aren't worried about technical enforcement of that (parties are trusted to report), then you can indeed just append something like
to a flow after the transaction is notarised. I'm actually changing and hopefully simplifying the transaction commit APIs at the moment and you'll probably be able to specify extra participants when committing/notarising a transaction.