After giving it some thought, I think Tor should use a Tor-specific protocol to interface with name plugins, not DNS.
My reasoning is as follows: the Tor daemon knows what it wants and is designed to source specific data from a name plugin. Where Tor specifies a custom protocol for this, this will match perfectly the type of queries and type of responses which Tor needs to ultimately obtain and convert to some corresponding internal in-memory representation.
As such, forcing this made-for-Tor format to be marshalled into a pre-existing format, namely that of DNS queries and responses, can only ever reduce the power and flexibility of the plugin interface. It only creates the potential for impedence discontinuities, and also creates a substantial nuisance and implementation barrier for plugins which are intended only for use with the Tor daemon. These plugins would need to source a DNS packet marshalling/unmarshalling library, which creates an unnecessary barrier to implementation, and both sides of the interface would be marshalling into a format which isn't especially aligned with the internal representations they'd ideally like to be speaking. As such, using DNS here feels rather pointless.
Looking at the Prop279 proposal as it stands, it would be trivial for a plugin that wants to work with DNS packets to convert a query to a DNS packet. As such, I see very little utility to adopting the DNS format for this.