I forked the Datacite plugin from Dulip and made a version for Crossref. It does not include automatic registration but has worked for us so far. Can not promise much support for that. I am waiting for the new DOI stuff PKP is preparing and will build a better one once they are ready.
Other plugins you might be interested in GitHub - ajnyga/quickSubmit at omp (just for OMP 3.2 at the moment)
And GitHub - ajnyga/autoApprovePublicationFormats if you are tired of clicking those publication format boxes green.