There is no way to trigger that from a block. The emailRows endpoint you mentioned is a private API endpoint and is not available for public use. However, custom blocks can make GET/POST requests to external APIs in general.
We don’t have any current plans to add “Send Record” functionality in the custom blocks SDK, but I’d be interested in hearing more about your use case.
Happy to help clarify - simply put, correctly spinning up a 3rd party service to notify users of things in their base introduces a decent amount of questions, risks, and extra effort (a la Sendgrid). Being able to email / notify users about their base is something I’d love to keep within Airtable to keep things secure and seamless. I imagine this could be a notifications API or something similar.