Eureka! I am able to successfully add the device via DNS name and it’s using udp6 for sure.
However, I don’t really want my LibreNMS setup to be dependent on DNS per se. I presume it’s doing DNS lookups at every poll/polling cycle after you tell it to add, instead of just at the time it’s added? Meh.
I attempted some trickery to add it with the brackets on cmd line, but it fails the ping because it appears to be using the brackets in the ping command. 
bash-5.1# lnms device:add --v2c -t udp6 -c test [dead:beef:dead:100::10]
Could not ping [dead:beef:dead:100::10]
bash-5.1# lnms device:add --v2c -t udp6 -c test \[dead:beef:dead:100::10\]
Could not ping [dead:beef:dead:100::10]