According to the web service API documentation, it is possible to pass in "CSR" in different formats to the certificateRequest call.
The CLI itself, though, currently supports only PKCS10, SPKAC, and CRMF. I.e. the "PUBLICKEY" request format is not supported.
It would be nice to have the CLI implement support for PUBLICKEY request type as well. This way more of the functionality would be available for testing using the web service CLI.