What are the arguments against running an authoritive nameserver (primary) e.g. bind in a ipv6 only environment? In the current setup the machine dedicated to the nameserver has only direct ipv6 connectivity (ipv4 only via NAT) - the secondary ns would be dualstack (the one from the hosting provider)
Strictly speaking there is nothing wrong with that configuration. In practice there are many poorly written resolver stacks in use in various applications.
You will notice that Hurricane Electric does not dual-stack ns1.he.net (ns1 is IPv4 only, ns2 through ns5 are fully dual-stacked). That is arguably an overly cautious approach.
I would not choose an IPv6-only host as a primary authoritative nameserver. However, I would feel comfortable having a dual-stacked primary and IPv6-only secondary (or tertiary, etc).