Please allow for multiple DNS resolver instances (with independently configured external servers, static entries, and cache).
The current single DNS resolver could just be 1 item in a list, to which others can be added.
These resolvers could be tied to internal interfaces using an interface list or they could listen on one or more addresses specified in their entry, whatever is more convenient.
Reason: you may want to use a different DNS service, like OpenDNS or another DNS with filtering capabilities, for your guest network.
Or you may want to have LAN systems resolve via a local DNS resolver like Windows Server and have the guest network only use internet DNS.