The clients that are listed as supporting ACMEv2 at
(including the latest release, 0.22, of Certbot which was announced yesterday) can get wildcard certificates from the staging (testing) ACMEv2 server
Remember that the certificates issued by the test server are not accepted by web browsers. They will produce an error message whenever someone visits your site. However, you can get experience with the way wildcard certificates from Let's Encrypt will work.