Currently, only one IPv6 address of a droplet may have a PTR record. Are you planning to add a feature to assign PTR records for all of the 15 available IPv6 addresses?
Looks like this question slipped through the cracks! For anyone stumbling across it now, support for PTR records for IPv6 addresses was actually added a while back. By default, the each Droplet with IPv6 enabled will be configured with a single IPv6 address. This is the address that will be used for any PTR records that will be generated for domains pointing at your server. Like with IPv4, if the hostname given to the Droplet is a FQDN, the PTR record will be generated automatically.
If you’re looking for more control over PTR records more generally, there is an open request for allowing users to manually set PTR records. Please vote for it on UserVoice if that’s something you’d like to see: