This provides for a quick means to deliver ipv6 access for ipv6 enabled hosts. Review this topology;
Here we have an unrouted ipv6 network prefix and will apply ipv6 NAT66 policies for the inside hosts located within the 2001:d8::/64 space.
A simple policy could be craft to allow the clients internally access to the ipv6 internet;
NOTE: pre fortios 5.x code does not have the "set nat enable" option available
NAT66 helps in a crunch and when you are limited or do not have enough ip6-prefixes.It's not ideal but will work in the same fashion of ipv4 SNAT'ing and provide an alternative means for local client access.
YMMV but with the size and availability of ipv6 prefixes, you should have NO requirement for SNATs imho.
NOTE: Firewall from cisco and juniper also allows for NAT66 & linux iptables allows for NAT66