How would I add a BCC to all outgoing email

If it's only emails that the users send (from the UI) - we can extend the MailApi and create a CustomMailApi in custom/modules/Emails/clients/base/api/CustomMailApi.php. In the handleMail method we can add the BCC in the request args. 

If you want to add emails for all outgoing emails (eg: auto replies, system emails, etc) - And since you say it's temporary - a non-upgrade safe approach is to directly modify modules/Emails/Email.php - and add your BCC whenever $mailer->send() is called. But since it's non upgrade safe - you can't do this in Cloud.

If it's only emails that the users send (from the UI) - we can extend the MailApi and create a CustomMailApi in custom/modules/Emails/clients/base/api/CustomMailApi.php. In the handleMail method we can add the BCC in the request args. 

If you want to add emails for all outgoing emails (eg: auto replies, system emails, etc) - And since you say it's temporary - a non-upgrade safe approach is to directly modify modules/Emails/Email.php - and add your BCC whenever $mailer->send() is called. But since it's non upgrade safe - you can't do this in Cloud.

I have a feeling thats the only way to do it.  I might be able to override the Email.php in the custom directory but I think its hardcoded too many times for that to work right.