Thanks to msfriedl, #OpenSSH now supports #ECDSA keys stored in PKCS#11 tokens such as #Yubico Yubikey 4. This is available in CVS/git HEAD now and will be in the OpenSSH 8.0 release.
(H/T djm@)
https://lists.mindrot.org/pipermail/openssh-unix-dev/2019-January/037468.html