Hi @Touma,
We are definitely planning to offer certs that use subjectAltName to cover multiple domains. In fact, support for this has been a major focus of client development, and is already working well in the client preview. A goal is to make it easy to reissue certificates quickly when you need to add or remove individual names.
I misunderstood the focus of your question and thought you were asking whether we could do exactly what PositiveSSL does. We can definitely do the multidomain part, just not the wildcard part!