I may have misunderstood what you are trying to acheive, here is how I understand it.
Host A on local network goes through WAN interface and sees OpenDNS setting apprioriate for WAN derived traffic.
Host B on local network goes through Opt1 interface and sees OpenDNS setting apprioriate for Opt1 derived traffic.
How I would acheive this first I would switch off the PFsense DNS forwarder.
next I would add the OpenDNS server IPs either statically or through DHCP to the end hosts.
Then have rules that constrain traffic for each host goes through which WAN.
You can still use failover if you wish to provide it for any services that do not require DNS lookups for initiation or during use, such as MSN, games etc….
If you have a lot of hosts I would go one step further and create an Alias list for the two groups and pop in the hosts that you want in each list.
Cheers
Paul