Question: Is there a command I can give the Monero daemon to calculate the total of all coinbase transactions so far?

How do block explorers calculate the Monero coin supply and how can I verify their accuracy?

Answer: 
There is no such command. However, it'd be easy to add. Feel free to add a request for this on <https://github.com/monero-project/bitmonero/issues>
This question has a fundamental misunderstanding of money supply and the link with coinbase transactions.

You would be right in a perfectly secure system, the coinbase transactions are the only way to add coins to the money supply. 

The reality however is different. Bugs in the protocol ([like the infamous keyimage bug that allowed the creation of infinite money](https://getmonero.org/2017/05/17/disclosure-of-a-major-bug-in-cryptonote-based-currencies.html)) can create additional coins in the system.

Creating commands like print\_coinbase\_tx\_sum may give you a good indication of the money supply but it should not be taken for facts.