RFC 4514 unfortunately does not define emailAddress, and I'm hesitant to add support for non-standard names to the RFC4514 parser. Email can still be added by manually specifying the OID: 1.2.840.113549.1.9.1=user@example.com which is compliant with the standard. The current version of ykman does not support this, but the next one (out soon!) will.

I know it's not as clear as emailAddress=... but at least it works.

Just to confirm, because I stumbled across the same exact issue today. And an hour's worth of Goolging 4514 never came up with a clear answer.

Does the current version of ykman support specifying the OID for email?

If so, what is the syntax for specifying OIDs; I've looked at Yubikey's docs for PIV and I can't seem to find any info on this.

Yes, the current version (5.0.0) supports this, using the syntax I provided in the last comment. The RFC itself can be found here: https://www.ietf.org/rfc/rfc4514.txt

Here's a full example of the syntax, showing the result using openssl:

ykman piv certificates generate 9a pub_key.pem -s "CN=Test,1.2.840.113549.1.9.1=user@example.com"

ykman piv certificates export 9a cert.pem
openssl x509 -in cert.pem -text

<snip>

Subject: emailAddress = user@example.com, CN = Test
...