Using "Intelligent SNAT" for outbound traffic?

this is a quite common question, with a little searching you should be able to find what you need.

this SOL gives a start: http://support.f5.com/kb/en-us/solutions/public/7000/800/sol7820.html

there they do it based on port, you want it on destination address, the iRule wiki (or again some searching) will be able to help with that.

if you got an irule ready post it for feedback.