Is it necessary to get every cert? Getting the CAs to participate would be the best way, but it seems there are workarounds that will result in a large number of certs to be listed, though not all of them:

"Google is currently operating a Certificate Transparency log, and we are filling the log with certificates that we retrieve while crawling the web. We are also actively working on monitoring and auditing software."

http://www.certificate-transparency.org/faq

Something like pinning for S/MIME would be great, too…

It doesn't require all of them. They just need enough to reach a critical mass at which point they can start rejecting.

Also, a client can submit their cert to the log regardless of CA support. The only thing CA support is needed for is automated submissions.