If you want to have an identd that can talk IPv6, you can choose oidentd.
If you are running it from inetd, you should configure your inetd to respond to IPv6 as well. I’m using openbsd-inetd, and the lines in /etc/inetd.conf to make it listen on both IPv4 and IPv6 for the auth service are:
auth stream tcp4 nowait root /usr/sbin/tcpd /usr/sbin/oidentd -I auth stream tcp6 nowait root /usr/sbin/tcpd /usr/sbin/oidentd -I
Note the protocol, which specifies IPv4 or IPv6. Also note the -I option to oidentd, to make it read from stdin and write to stdout and to exit after answering one request (needed for inetd operation).
I briefly considered nullidentd, but the description made it sound like it would only ever return one static string. Not quite what I was looking for, and I didn’t investigate further.