I don't see why the redirect (i.e. an actual 302) is going to still happen. In theory the LTM could intercept a 302 and act upon it itself, but if you can anticipate that situation, you'd just do a rewrite in the first place.
so the iRule just says if the user agent contains "mozilla" or "testagent" then send the request to the pool aaa, and change the HTTP Host header to suit what the end server is expecting. otherwise send it to pool zzz. I put the Host header change in as an example, maybe you actually want to change the uri as well or something. If pool zzz is the default pool on the vip then there's no need for the else {} block at all.
Nimbostratus