A WIP doesn't perform forwarding to a virtual server. It only answers with the DNS query with the correct IP address.
Natting of your public ip to RFC1918 should be done by a firewall (or BIG-IP).
Having RFC1918 address on your virtual servers and WIP serving public IP can be done by creating a dummy virtual server with the public IP address and use this virtual server in your wide ip pool.
I don't filly understand the question. Are you asking if you can answer a DNS query with a public answer when outside and private when inside? For use when a VPN is in place, for example. This is a use case that I use often. I use a DNS iRule and respond with a specific answer when certain conditions are met such as the IP address pool for my VPN.