This is definitely doable, assuming the username you're talking about using to base your redirects on is readable in the request. All you'd need to do, as Chris alluded to, would be to create a data group for the IPs that you want to redirect. The key would be the IP you want to redirect, the value of each entry would be what host/IP you want them redirected to. You'd then need to create another data group with user names in it. All you'd need then is to create an iRule that does a findclass on the first datagroup to see if the IP is matched and grab the associated value (the new IP/host to redirect the user to), then perform a matchclass on the data group containing usernames. If the user isn't present, redirect to the newly queried host/IP and you should be set.
Let us know if you need help with any of the specific steps in the code.
Nimbostratus