[jeroenhd]

libupnp/miniupnp seem to support IPv6 pinholes just fine through WANIPv6FirewallControl. I don't sell routers so I don't know how up to date the libraries on your average router firmware are (most likely "ancient") but this isn't a protocol problem. If you run firmware like OpenWRT you've had support for it for at least five years now.

Most consumer routers I've seen these days come with UPNP disabled by default, though.

For most P2P traffic (which includes a third party handshake server) you can probably skip port forwarding entirely; just send UDP packets both ways and the firewall will figure it out. For stateful protocols like TCP and friends (SCTP etc.) that's harder to accomplish this, that's where you need pinholing.

It's possible that your router simply doesn't support IPv6 pinholing but I think the more likely scenario for breakages is that client software doesn't bother implementing it.