It can't be what they do already because Let's Encrypt does not currently operate an ECDSA intermediate. This document describes a hierarchy they've just recently issued, and which is not yet in production use.
If you present a CSR for an ECDSA public key today, Let's Encrypt will issue a certificate signed by their RSA intermediate Let's Encrypt Authority X3, for your ECDSA key.
They haven't actually specified whether you'll get certificates in the new ECDSA hierarchy from the same API endpoint or need to use a different endpoint.
If you present a CSR for an ECDSA public key today, Let's Encrypt will issue a certificate signed by their RSA intermediate Let's Encrypt Authority X3, for your ECDSA key.
They haven't actually specified whether you'll get certificates in the new ECDSA hierarchy from the same API endpoint or need to use a different endpoint.