Using link addresses is highly inadvisable, but.. The PTR records use all the digits of the IPv6 address dotted together. For your address [fe80::20f:feff:fe10:ece0], the line would look like this:
0.e.c.e.0.1.e.f.f.f.e.f.f.0.2.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.e.f.ip6.int IN PTR hostname.example.com.
With that said, you should get a /64 or /48 from a tunnel broker such as Freenet6. They do UDP tunnels now too, which should pierce through most firewalls.