Ok, after a few days of leaving it alone to see if it would update, it looks like I got HE/Tunnelbroker's DynDNS to work:
Used the "custom" Dynamic DNS choice and the custom update URL is
http://[USERNAME]:[PASSWORD]@ipv4.tunnelbroker.net/nic/update?hostname=[DOMAIN]&myip=[IP]"
Hostname=Tunnel ID number (get this by logging into tunnelbroker.net, then click the tunnel, Look on the IPv6 Tunnel tab, near the top)
Username = tunnelbroker.net username
Password=the update password listed in the tunnel's Advanced tab on tunnelbroker.net or if none is listed, your tunnelbroker.net password (less secure).
...The rest I left with the defaults.
To verify that it worked, you can use the following command after ssh'ing into the A+:
sh /usr/lib/ddns/dynamic_dns_updater.sh Tunnelbroker
(I named mine "Tunnelbroker" but you would type whatever you named the service when you clicked Add)
It should report Update Status with "good" and your current IP.