Hi Chris -
I used to use the bigip to frontend proxy servers used for outbound internet access. This idea came across my mind a few times, but I never got to implement that before I left. I was thinking of using a rule, not persistance - because as you said, the number of GETs is extremely huge.
I was thinking of using the first character of the requested domain as the deciding factor in my rule. So, for example, I would send all requests for domains starting with a-m to pool1, which has cache server1 as a higher priority device, while all requests to n-z would go to pool2, which has cache server2 as a higher priority. Obviously the other box would be in the pool as well, but at a lower priority in case of failure.
Does that make sense? Would that work for you?
Al