Please build openvpn at least in the openssl case with --enable-x509-alt-username. The support is documented in the manpage without information that it will never work.
I will add this as a build-time port option.
Port changes are currently being tested in poudriere.
A commit references this bug:
Date: Wed Mar 25 20:06:21 UTC 2015
New revision: 382265
Add a X509ALTUSERNAME port option to enable the --x509-username-field
Submitted by: email@example.com