Split DNS for Explicit Proxy

I have a Fortigate 600B that is used as a web proxy server. For name resolution the Fortigate uses one of our corporate DNS servers. It also has a small number of zones in its local DNS database. My issue is that now for one of the zones I require the Fortigate to look at its internal database for some entries but, if there are no specific host entries in the local zone, then do a lookup on our corporate DNS server. At the moment if the host entry is not found in the zone in its local DNS then the lookup bombs. I know that a split DNS configuration can be done for user DNS lookups but I can' t get this working for the lookups required by the explicit proxy. Any ideas?