VS> The Transmission torrent client, and the syncthing file VS> synchronization utility can use the UPnP protocol to request a VS> firewall to pass *IPv4* incoming traffic (and create a port porwarding VS> for IPv4 NAT). They cannot however (at least to my knowledge) use UPnP VS> or any other protocol to request a router to open a hole for incoming VS> traffic in an *IPv6* firewall.
I see. Or so I think. You ask for some kind of "IPv6 equivalent" for UPnP. But why would you want that? UpNP is a questionable idea anyway. For IPv4 it creates an entry in de NAT table and as a side effect creates a hole in the firewall.
But why would you need that for IPv6?
For IPv6 there (normally) is no NAT, so no need to create an entry in a NAT table. In IPv6 avery device has a Unique Global Address, so one can simply create pinholes in advance as needed for the address in question.
