Letsencrypt won't issue certificates to IP addresses - but will to domain names (including those assigned by dynamic DNS providers, but most dynamic DNS providers need manual sign-up with username and password)
Of course, has-a.name will be rate-limited to 50 certificate a week by Letsencrypt until they get themselves onto the public suffix list. And whether it's a good idea to bypass LE's no-certs-for-ip-addresses policy is another matter...